mesa: Rewrite shader-based texture image state updates.

mesa: Rewrite shader-based texture image state updates.

Instead of walking 6 shader stages for each of the 96 combined texture
image units, now we just walk the samplers used in each shader stage.

With cairo-perf-trace on Xephyr with glamor, I'm seeing a -6.50518% +/-
2.55601% effect on runtime (n=22) since the "drop _EnabledUnits" change.
No significant performance difference on an apitrace of minecraft (n=442).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>