i965: Fix incorrect maximum PS thread count shift on Ivybridge.

i965: Fix incorrect maximum PS thread count shift on Ivybridge.

At one point, the documentation said that max thread count in 3DSTATE_PS
was at bit offset 23, but it's actually 24 on Ivybridge.  Not only did
this halve our thread count, it caused us to write 1 into a bit 23, which
is marked as MBZ (must be zero).  Furthermore, it made us write an even
number into this field, which is apparently not allowed.  Apparently we
were just lucky it worked.

NOTE: This is a candidate for the 7.11 branch.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>