i965/vp: Use the sampler for pull constant loads on Gen7/7.5.

i965/vp: Use the sampler for pull constant loads on Gen7/7.5.

This improves performance in Trine 2 at 1280x720 (windowed) on "Very
High" settings by 30% (in the interactive menu) to 45% (in the forest
by the giant frog) on Haswell GT3e.

It also now generates the same assembly on Gen7 as it does on Gen8,
which always used the sampler for both types.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Matt Turner <mattst88@gmail.com>