i965: Update URB partitioning code for Haswell's GT3 variant.

i965: Update URB partitioning code for Haswell's GT3 variant.

Haswell's GT3 variant offers 32kB of URB space for push constants, while
GT1 and GT2 match Ivybridge, providing 16kB.  Update the code to reserve
the full 32kB on GT3.

v2: Specify push constant size correctly.  I thought GT3 reinterpreted
    the value as multiples of 2kB, but it doesn't.  You simply have to
    program an even number.

NOTE: This is a candidate for stable branches.

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