i965/fs: Allocate more register classes on gen7.

i965/fs: Allocate more register classes on gen7.

For texturing from GRFs, we now have payloads of arbitrary sizes up to the
message length limit.

v2 (Kenneth Graunke): Rebase on intel_context -> brw_context change.
v3: Add some comment text.
v4: Change some magic 16s to BRW_MAX_MRF (noted by Ken).  Leave the 11,
    which is the magic "max sampler message length".  BRW_MAX_MRF sizing
    on the little int arrays is retained because I could see us needing to
    extend in the future if we move to GRFs for FB writes (those go to at
    least 12 long in a quick scan of the specs)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v2)
Acked-by: Matt Turner <mattst88@gmail.com>