i965/fs: Remove force_sechalf stack

i965/fs: Remove force_sechalf stack

Only Gen4 color write setup uses the force_sechalf flag, and it only
sets it on a single instruction.  It also already has to get a pointer
to the instruction and manually set the saturate flag, so we may as well
just set force_sechalf the same way and avoid the complexity of a stack.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Acked-by: Anuj Phogat <anuj.phogat@gmail.com>