intel/fs: Fix FB write inst groups

intel/fs: Fix FB write inst groups

This commit does two things.  First, it simplifies the way we compute
the FB write group bit.  There's no reason to use a ternary because
inst->group / 16 can only be 0 or 1.  Second, it fixes an order-of-
operations bug where the ternary wasn't selecting between (1 << 11) and
0 but between (1 << 11) and 0 | brw_dp_write_desc(...).

Fixes: 0d9648416 "intel/compiler: Use generic SEND for Gen7+ FB writes"
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>