i965/fs: Implement scratch read/write support for Broadwell.

i965/fs: Implement scratch read/write support for Broadwell.

To make sure that both the Gen4 and Gen7 style messages work, I
initially disabled the SHADER_OPCODE_GEN7_SCRATCH_READ optimization,
ran Piglit, re-enabled it, and ran Piglit again.  Both worked fine.

Fixes 40 Piglit tests (most of the varying-packing category).

v2: Move num_regs assertion from gen8_fs_generator to
    gen8_set_dp_scratch_message() (suggested by Eric).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>