intel/compiler: fix ddy for half-float in Broadwell

intel/compiler: fix ddy for half-float in Broadwell

Broadwell has restrictions that apply to Align16 half-float that
make the Align16 implementation of this invalid for this platform.
Use the gen11 path for this instead, which uses Align1 mode.

The restriction is not present in cherryview, gen9 or gen10, where
the Align16 implementation seems to work just fine.

v2:
 - Rework the comment in the code, move the PRM citation from the
   commit message to the comment in the code (Matt)
 - Cherryview isn't affected, only Broadwell (Matt)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net> (v1)
Reviewed-by: Matt Turner <mattst88@gmail.com>