i965: Allocate space in the binding table for non-coherent FB fetch.

i965: Allocate space in the binding table for non-coherent FB fetch.

Unfortunately due to the inconsistent meaning of some surface state
structure fields, we cannot re-use the same binding table entries for
sampling from and rendering into the same set of render buffers, so we
need to allocate a separate binding table block specifically for
render target reads if the non-coherent path is in use.

The slight noise is due to the change of
brw_assign_common_binding_table_offsets to return the next available
binding table index rather than void.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>