radeonsi: Don't modify PA_SC_RASTER_CONFIG register value if rb_mask == 0

radeonsi: Don't modify PA_SC_RASTER_CONFIG register value if rb_mask == 0

E.g. this could happen on older kernels which don't support the
RADEON_INFO_SI_BACKEND_ENABLED_MASK query yet. The code in
si_write_harvested_raster_configs() doesn't deal with this correctly and
would probably mangle the value badly.

Cc: "10.4 10.3" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>