drm/radeon: expose render backend mask to the userspace

This will allow userspace to correctly program the PA_SC_RASTER_CONFIG
register, so it can be considered a fix.

Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org