drm/amd/display: Fix a merge on amdgpu_dm_plane.c

In mainline commit 9cf26c89 ("Merge drm/drm-next into
drm-misc-next") fixed up amdgpu_dm_plane.c, but in
amd-staging-drm-next this refers to the wrong function.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>