i965/fs: Refactor the texture emission logic into a single function.

i965/fs: Refactor the texture emission logic into a single function.

Before, we had 3 different emit functions for various different gen's,
as well as some ancilliary work that was the same across all gen's which
was either contained in functions or duplicated across the GLSL IR and
Mesa IR backends. Now, we have a single method, emit_texture(), that
takes all the information needed to make a texture instruction and
handles all the setup, and all we have to do to emit a texture
instruction while converting from GLSL IR, Mesa IR, or any new backend
is to extract the information emit_texture() needs and then call it.

v2: Significant rebasing (by Ken).

Signed-off-by: Connor Abbott <connor.abbott@intel.com>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>