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>