i965/gen7: Increase the WM threads to hardware limits.

i965/gen7: Increase the WM threads to hardware limits.

This thread count is only supposed to be enabled when "WIZ Hashing Disable in
GT_MODE register enabled."  I've always been confused whether that means the
bit in the register should be 1 or 0.  For my IVB GT2's register 0x7008 value
of 0x0, this appears to work fine.

Improves l4d2 performance at 640x480 by 0.88 +/- 0.11% (n=88).  Improves
performance with rasterization at 1280x1024 by 1.45% +/- 0.36% (n=6).

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