i965/fs: Implement fs_inst::force_sechalf support on Broadwell.

i965/fs: Implement fs_inst::force_sechalf support on Broadwell.

Back when I originally wrote this code, force_sechalf was only used for
Gen4 code, so I didn't bother hooking it up.  However, it's used more
generally these days.  In particular, we use it for computing
gl_SamplePosition.

Fixes Piglit's spec/ARB_sample_shading/builtin-gl-sample-position tests.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=77222
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>