i965: Fix predicated-send-based discards with MRT.

i965: Fix predicated-send-based discards with MRT.

We need the header setup to not be predicated on which pixels are
undiscarded.  I'm not sure originally if I had thought that the mask
disable implied predicate disable, or if I had just misread the mask
disable as predicate disable.  Either way, I know I had spent more time
thinking about this in the gen8 generator than the gen7 generator.

Plus, it turns out that I had mis-implemented the "the GPU will use the
predicate unless this header is present" comment, by skipping setting up
the pixel mask when the header was present.

Fixes GPU hangs in piglit glsl-fs-discard-mrt, Trine, Trine 2 and
preusmably MLL.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=75207
Tested-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>