drm/radeon: protect against div by 0 in backend setup

Make sure at least one RB is enabled in
r6xx_remap_render_backend() to avoid an division by
zero in some corner cases.

See:
https://bugzilla.redhat.com/show_bug.cgi?id=892233



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