drm/amd/display: remove variable "result" in dcn20_patch_unknown_plane_state()

Fix the following coccicheck warning:

drivers/gpu/drm/amd/display/dc/dcn20/dcn20_resource.c:3216:16-22:
Unneeded variable: "result". Return "DC_OK" on line 3229

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>