drm/amd/display: Don't expose support for DRM_FORMAT_RGB888

[Why]
This format isn't supported in DC and some IGT tests fail since we
expose support for it.

[How]
Remove it.
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -3777,7 +3777,6 @@ static const struct drm_plane_helper_funcs dm_plane_helper_funcs = {
  * check will succeed, and let DC implement proper check
  */
 static const uint32_t rgb_formats[] = {
-	DRM_FORMAT_RGB888,
 	DRM_FORMAT_XRGB8888,
 	DRM_FORMAT_ARGB8888,
 	DRM_FORMAT_RGBA8888,