i965/gen7: Relax restrictions on fake MRFs

i965/gen7: Relax restrictions on fake MRFs

Gen6 has write-only MRF registers, and for ease of implementation we
paritition off 16 general purposes registers to act as MRFs on Gen7.

Knowing that our Gen7 MRFs are actually GRFs, we can do things we can't
do with real MRFs:
   - read from them;
   - return values directly to them from a send instruction; and
   - compute directly to them with math instructions.

Reviewed-by: Eric Anholt <eric@anholt.net>