i965/fs: Rename "sampler" to "texunit" in texturing code.

i965/fs: Rename "sampler" to "texunit" in texturing code.

The number we're passing around is actually the ID of the texture unit,
as opposed to the numerical value our of sampler uniforms.  Calling it
"texunit" clarifies this slightly.

Don't bother renaming fs_instruction::sampler.  Although it's currently
the texture unit, this series will change that.  No need for the churn.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>