i965/fs: Combine generate_math[12]_gen6 methods.

i965/fs: Combine generate_math[12]_gen6 methods.

These used to call different math emitters (brw_math vs. brw_math2).
Now that they both call gen6_math, they're virtually identical.

When unrolling SIMD16 to multiple SIMD8 operations, we should take care
not to apply sechalf to brw_null_reg for src1.  Otherwise, we'd end up
with BRW_ARF_NULL + 1 as the register number, and I'm not sure if that's
valid.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>