i965/fs: Support arbitrarily large sampler state indices on Haswell+.

i965/fs: Support arbitrarily large sampler state indices on Haswell+.

The message descriptor's "Sampler Index" field is only 4 bits (on all
generations of hardware), so it can only represent indices 0 through 15.

Haswell introduced a new field in the message header - "Sampler State
Pointer".  Normally, this is copied straight from g0, but we can also
add a byte offset (as long as it's a multiple of 32).

This patch uses a "Sampler State Pointer" offset to select a group of
16 sampler states, and then uses the "Sampler Index" field to select
the state within that group.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>