i965/blorp: Use 16 pixel dispatch on Gen7.

i965/blorp: Use 16 pixel dispatch on Gen7.

Gen7 hardware requires us to enable at least one WM dispatch mode,
even if there is no program being dispatched to.  When this code was
only used for HiZ operations (which don't use a WM program), we used
32-pixel dispatch, because it didn't matter.  But blit programs are
compiled for 16-pixel dispatch.  So just enable 16-wide dispatch
unconditionally.

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
v2: Enable 16-wide dispatch unconditionally rather than add the
unnecessary complication of using 32-wide dispatch when there is no WM
program.