drm/amd/display: Extract 3rd plane from metadata

Now that we actually expose the right number of planes we get
problems with modifier aware libva if we use an unsupported modifier.

So this ensures that we as much as possible use a modifier that
radeonsi can actually render to without going to extreme difficulty
to synchronize an extra DCC metadata plane with implicit sync.

Looking into the opaque metadata isn't ideal but this converts to
something mainly for the purpose of the userspace driver and falls
back properly if it isn't what we expect.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>