This patch is to fix the NULL pointer that missed set_disp_pattern_generator callback on DCN301

[  505.054167] BUG: kernel NULL pointer dereference, address: 0000000000000000
[  505.054176] #PF: supervisor instruction fetch in kernel mode
[  505.054181] #PF: error_code(0x0010) - not-present page
[  505.054185] PGD 0 P4D 0
[  505.054199] Oops: 0010 [#1] SMP NOPTI
[  505.054211] CPU: 6 PID: 1306 Comm: modprobe Tainted: G        W  OE     5.9.0-rc5-custom #1
[  505.054216] Hardware name: AMD Chachani-VN/Chachani-VN, BIOS WCH0A29N_RAPV16.FD 10/29/2020
[  505.054225] RIP: 0010:0x0
[  505.054234] Code: Bad RIP value.
[  505.054239] RSP: 0018:ffffb88541c66f60 EFLAGS: 00010206
[  505.054245] RAX: 0000000000000000 RBX: ffff912836070000 RCX: 0000000000000003
[  505.054248] RDX: 000000000000000c RSI: ffff9128365001e8 RDI: ffff912836070000
[  505.054252] RBP: ffffb88541c66fd8 R08: 0000000000000002 R09: ffffb88541c66fa2
[  505.054265] R10: 0000000000009580 R11: 0000000000000008 R12: ffff9128365001e8
[  505.054272] R13: 000000000000000c R14: 0000000000000438 R15: ffff9128a48bd000
[  505.054279] FS:  00007f09f999f540(0000) GS:ffff9128b3f80000(0000) knlGS:0000000000000000
[  505.054284] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  505.054288] CR2: ffffffffffffffd6 CR3: 00000002db98c000 CR4: 0000000000350ee0
[  505.054291] Call Trace:
[  505.055024]  dcn20_blank_pixel_data+0x148/0x260 [amdgpu]
[  505.055730]  dcn20_enable_stream_timing+0x381/0x47c [amdgpu]
[  505.056641]  dce110_apply_ctx_to_hw+0x337/0x577 [amdgpu]
[  505.056667]  ? put_object+0x2f/0x40
[  505.057329]  dc_commit_state+0x4b3/0x9d0 [amdgpu]
[  505.058030]  amdgpu_dm_atomic_commit_tail+0x405/0x1ec6 [amdgpu]
[  505.058053]  ? update_stack_state+0x103/0x170
[  505.058071]  ? __module_text_address+0x12/0x60
Signed-off-by: Huang Rui <ray.huang@amd.com>
Tested-by: Changfeng <Changfeng.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>