[Why]
Raven has support for combining pipes for DRM_PLANE_TYPE_OVERLAY use
but no overlays are exposed to userspace.

[How]
Expose overlay planes based on DC plane caps.

If all the pipes are in use then the atomic commits can fail, but this
is expected behavior for userspace.

Only support RGB on overlays for now.
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>