i965: Use the correct number of threads for compute shaders.

i965: Use the correct number of threads for compute shaders.

We were programming the number of threads per subslice, when we should
have been programming the total number of threads on the GPU as a whole.

Thanks to Curro and Jordan for helping track this down!

On Skylake GT3e:
- Improves performance in Unreal's Elemental Demo by roughly 1.5-1.7x.
- Improves performance in Synmark's Gl43CSDof by roughly 3.7x.
- Improves performance in Synmark's Gl43GSCloth by roughly 1.18x.

On Broadwell GT2:
- Improves performance in Unreal's Elemental Demo by roughly 1.2-1.5x.
- Improves performance in Synmark's Gl43CSDof by roughly 2.0x.
- Improves performance in Synmark's Gl43GSCloth by 1.47035% +/-
  0.255654% (n=25).

On Haswell GT3e:
- Improves performance in Unreal's Elemental Demo (in GL 4.3 mode)
  by roughly 1.10x.
- Improves performance in Synmark's Gl43CSDof by roughly 1.18x.
- Decreases performance in Synmark's Gl43CSCloth by -1.99484% +/-
  0.432771% (n=64).

On Ivybridge GT2:
- Improves performance in Unreal's Elemental Demo (in GL 4.2 mode)
  by roughly 1.03x.
- Improves performance in Synmark's G/43CSDof by roughly 1.25x.
- No change in Synmark's Gl43CSCloth (n=28).

Cc: "12.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>