i965/fs: Switch to using sampler LD messages for uniform pull constants.

i965/fs: Switch to using sampler LD messages for uniform pull constants.

When forcing the compiler to always generate pull constants instead of
push constants (in order to have an easy to use testcase), improves
performance of my old GLSL demo 23.3553% +/- 1.42968% (n=7).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=60866
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>