i965/gen6/gs: Implement GS_OPCODE_URB_WRITE_ALLOCATE.

i965/gen6/gs: Implement GS_OPCODE_URB_WRITE_ALLOCATE.

Gen6 geometry shaders need to allocate URB handles for each new vertex they
emit after the first (the URB handle for the first vertex is obtained via the
FF_SYNC message).

This opcode adds the URB allocation mechanism to regular URB writes.

Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>