drm/rockchip: fix VOP vblank race

We have seen a case of a bad reference count for vblanks with the
Rockchip VOP:

	------------[ cut here ]------------
	WARNING: CPU: 1 PID: 383 at drivers/gpu/drm/drm_irq.c:1198 drm_vblank_put+0x40/0xcc
	Modules linked in: brcmfmac brcmutil
	CPU: 1 PID: 383 Comm: kworker/u8:2 Not tainted 4.9.75-rt60 #1
	Hardware name: Rockchip (Device Tree)
	Workqueue: events_unbound flip_worker
	Backtrace:
	[<c010b7b0>] (dump_backtrace) from [<c010ba4c>] (show_stack+0x18/0x1c)
	 r7:c0b1b13c r6:600b0013 r5:00000000 r4:c0b1b13c
	[<c010ba34>] (show_stack) from [<c032d248>] (dump_stack+0x78/0x94)
	[<c032d1d0>] (dump_stack) from [<c011e6e8>] (__warn+0xe4/0x104)
	 r7:00000009 r6:c03cf26c r5:00000000 r4:00000000
	[<c011e604>] (__warn) from [<c011e7c0>] (warn_slowpath_null+0x28/0x30)
	 r9:eeb443a0 r8:eeb443c8 r7:ee8a5ec0 r6:ee8a5ec0 r5:edb47f00 r4:ee096200
	[<c011e798>] (warn_slowpath_null) from [<c03cf26c>] (drm_vblank_put+0x40/0xcc)
	[<c03cf22c>] (drm_vblank_put) from [<c03cf310>] (drm_crtc_vblank_put+0x18/0x1c)
	 r5:edb47f00 r4:ee3c8a80
	[<c03cf2f8>] (drm_crtc_vblank_put) from [<c03ef9b4>] (vop_fb_unref_worker+0x18/0x24)
	[<c03ef99c>] (vop_fb_unref_worker) from [<c03df194>] (flip_worker+0x98/0xb4)
	 r5:edb47f00 r4:eeb443a8
	[<c03df0fc>] (flip_worker) from [<c0134808>] (process_one_work+0x1a8/0x2fc)
	 r9:00000000 r8:ee807d00 r7:00000000 r6:ee809c00 r5:eeb443a8 r4:edfe5f80
	[<c0134660>] (process_one_work) from [<c01358ec>] (worker_thread+0x2ac/0x458)
	 r10:00000088 r9:edfe5f98 r8:ee809c2c r7:c0b04100 r6:ee809c00 r5:ee809c00
	 r4:edfe5f80
	[<c0135640>] (worker_thread) from [<c013a0bc>] (kthread+0xfc/0x10c)
	 r10:00000000 r9:00000000 r8:c0135640 r7:edfe5f80 r6:00000000 r5:edf0e240
	 r4:ee8a4000 r3:ed194e00
	[<c0139fc0>] (kthread) from [<c0107cb8>] (ret_from_fork+0x14/0x3c)
	 r8:00000000 r7:00000000 r6:00000000 r5:c0139fc0 r4:edf0e240
	---[ end trace 0000000000000002 ]---

It seems that this is caused by unfortunate timing between
vop_crtc_atomic_flush() and vop_handle_vblank() given the following
ordering:

	atomic_flush		handle_vblank
	------------		-------------

	drm_flip_work_queue
	set_bit
	     			if (test_and_clear_bit(...))
	     				drm_flip_work_commit
	drm_vblank_get

This results in vop_fb_unref_worker (called as flip work) decrementing
the vblank refcount before it has been incremented.
Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Sandy huang <hjc@rock-chips.com>
Signed-off-by: Sandy Huang <hjc@rock-chips.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180328160351.23763-1-john@metanate.com

drivers/gpu/drm/rockchip/rockchip_drm_vop.c

@@ -1017,9 +1017,9 @@ static void vop_crtc_atomic_flush(struct drm_crtc *crtc,
 			continue;
 
 		drm_framebuffer_get(old_plane_state->fb);
+		WARN_ON(drm_crtc_vblank_get(crtc) != 0);
 		drm_flip_work_queue(&vop->fb_unref_work, old_plane_state->fb);
 		set_bit(VOP_PENDING_FB_UNREF, &vop->pending);
-		WARN_ON(drm_crtc_vblank_get(crtc) != 0);
 	}
 }