swr/rast: Fix read-back of render target array index

swr/rast: Fix read-back of render target array index

The last FE stage can emit render target array index. Currently we only
check to see if GS is emitting it. Moved the state to BACKEND_STATE and
plumbed the driver to set it.

Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>