i965/bxt: Fix conservative wm thread counts.

i965/bxt: Fix conservative wm thread counts.

When setting the conservative thread counts, I halved everything. That isn't
correct for the wm, which has nothing to do with actual thread counts. I suck.

BXT only has 1 slice, and there is some ambiguity about subslices, so just
reserve the max possible for now. It looks like this might fix:
piglit.spec.glsl-1_50.execution.variable-indexing.gs-output-array-vec4-index-wr.bxtm64.
I kind of question why that is, but it is what Jenkins says.

Mark is current running some of the other blacklisted tests on this patch. (it
effects anything requiring scratch space).

Cc: mesa-stable <mesa-stable@lists.freedesktop.org>
Cc: Neil Roberts <neil@linux.intel.com>
Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Tested-by: Mark Janes <mark.a.janes@intel.com>