drm/amd/display: remove set but not used variable 'old_plane_crtc'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c: In function dm_determine_update_type_for_commit:
drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c:6516:36: warning: variable old_plane_crtc set but not used [-Wunused-but-set-variable]

It is introduced by commit a87fa993 ("drm/amd/display:
Build stream update and plane updates in dm"), but never used,
so remove it.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>