i965/fs: Use the actual hardware g0 register for texel offset setup.

i965/fs: Use the actual hardware g0 register for texel offset setup.

The idea here is to set up the message header with the Sampler State
pointer which the hardware provides as part of the PS Thread Payload in
register g0.

Unfortunately, the existing code

   fs_reg(GRF, 0, BRW_REGISTER_TYPE_UD))

actually references "virtual GRF 0" rather than the hardware g0.  This
is just some arbitrary GRF temporary which will get register allocated.

So, we ended up setting up the header with garbage.

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