intel/fs: Use helper for discard sample mask flag subregister number.

intel/fs: Use helper for discard sample mask flag subregister number.

Use it instead of hard-coding f0.1 for the sample mask of programs
that use discard.  This will make the task easier when we replace f0.1
with another flag register location in order to support discard with
SIMD32 shaders.

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