The atomic check hooks must look up the encoder to be used with a
connector from the connector's atomic state, and not assume that it's
the connector's current attached encoder. The latter one can change
under the atomic check func, or can be unset yet as in the case of MST
connectors.

This fixes
[    7.940719] Oops: 0000 [#1] SMP NOPTI
[    7.944407] CPU: 2 PID: 143 Comm: kworker/2:2 Not tainted 5.6.0-1023-oem #23-Ubuntu
[    7.952102] Hardware name: Dell Inc. Latitude 7320/, BIOS 88.87.11 09/07/2020
[    7.959278] Workqueue: events output_poll_execute [drm_kms_helper]
[    7.965511] RIP: 0010:intel_psr_atomic_check+0x37/0xa0 [i915]
[    7.971327] Code: 80 2d 06 00 00 20 74 42 80 b8 34 71 00 00 00 74 39 48 8b 72 08 48 85 f6 74 30 80 b8 f8 71 00 00 00 74 27 4c 8b 87 80 04 00 00 <41> 8b 78 78 83 ff 08 77 19 31 c9 83 ff 05 77 19 48 81 c1 20 01 00
[    7.977541] input: PS/2 Generic Mouse as /devices/platform/i8042/serio1/input/input5
[    7.990154] RSP: 0018:ffffb864c073fac8 EFLAGS: 00010202
[    7.990155] RAX: ffff8c5d55ce0000 RBX: ffff8c5d54519000 RCX: 0000000000000000
[    7.990155] RDX: ffff8c5d55cb30c0 RSI: ffff8c5d89a0c800 RDI: ffff8c5d55fcf800
[    7.990156] RBP: ffffb864c073fac8 R08: 0000000000000000 R09: ffff8c5d55d9f3a0
[    7.990156] R10: ffff8c5d55cb30c0 R11: 0000000000000009 R12: ffff8c5d55fcf800
[    7.990156] R13: ffff8c5d55cb30c0 R14: ffff8c5d56989cc0 R15: ffff8c5d56989cc0
[    7.990158] FS:  0000000000000000(0000) GS:ffff8c5d8e480000(0000) knlGS:0000000000000000
[    8.047193] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    8.052970] CR2: 0000000000000078 CR3: 0000000856500005 CR4: 0000000000760ee0
[    8.060137] PKRU: 55555554
[    8.062867] Call Trace:
[    8.065361]  intel_digital_connector_atomic_check+0x53/0x130 [i915]
[    8.071703]  intel_dp_mst_atomic_check+0x5b/0x200 [i915]
[    8.077074]  drm_atomic_helper_check_modeset+0x1db/0x790 [drm_kms_helper]
[    8.083942]  intel_atomic_check+0x92/0xc50 [i915]
[    8.088705]  ? drm_plane_check_pixel_format+0x4f/0xb0 [drm]
[    8.094345]  ? drm_atomic_plane_check+0x7a/0x3a0 [drm]
[    8.099548]  drm_atomic_check_only+0x2b1/0x450 [drm]
[    8.104573]  drm_atomic_commit+0x18/0x50 [drm]
[    8.109070]  drm_client_modeset_commit_atomic+0x1c9/0x200 [drm]
[    8.115056]  drm_client_modeset_commit_force+0x55/0x160 [drm]
[    8.120866]  drm_fb_helper_restore_fbdev_mode_unlocked+0x54/0xb0 [drm_kms_helper]
[    8.128415]  drm_fb_helper_set_par+0x34/0x50 [drm_kms_helper]
[    8.134225]  drm_fb_helper_hotplug_event.part.0+0xb4/0xe0 [drm_kms_helper]
[    8.141150]  drm_fb_helper_hotplug_event+0x1c/0x30 [drm_kms_helper]
[    8.147481]  intel_fbdev_output_poll_changed+0x6f/0xa0 [i915]
[    8.153287]  drm_kms_helper_hotplug_event+0x2c/0x40 [drm_kms_helper]
[    8.159709]  output_poll_execute+0x1aa/0x1c0 [drm_kms_helper]
[    8.165506]  process_one_work+0x1e8/0x3b0
[    8.169561]  worker_thread+0x4d/0x400
[    8.173249]  kthread+0x104/0x140
[    8.176515]  ? process_one_work+0x3b0/0x3b0
[    8.180726]  ? kthread_park+0x90/0x90
[    8.184416]  ret_from_fork+0x1f/0x40

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2361
References: https://gitlab.freedesktop.org/drm/intel/-/issues/2486Reported-by: William Tseng <william.tseng@intel.com>
Reported-by: Cooper Chiou <cooper.chiou@intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201027160928.3665377-1-imre.deak@intel.com