i965/skl: Set mask bits in PIPELINE_SELECT on Skylake.

i965/skl: Set mask bits in PIPELINE_SELECT on Skylake.

Skylake has some extra bits in PIPELINE_SELECT, none of which are
interesting for a 3D driver.  In order to selectively change them, it
also introduces new "mask bits" in 15:8.  We care about the "Pipeline
Selection" bits (1:0), so set the mask to 0x3.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>