i965: Fix GPU hang related to multiple render targets and alpha testing

i965: Fix GPU hang related to multiple render targets and alpha testing

This patch should have been the part of commit e592f7df.
In a situation when there are multiple render targets with alpha testing
enabled, if fragment shader doesn't write to draw buffer zero, it causes
the GPU hang on SKL. No GPU hang is seen on HSW. Simulator gives a
warning for all gen6+ h/w:
"Illegal render target write message length 0xa expected 0xc"

This patch fixes the GPU hang as well as the simulator warning with
new piglit test fbo-mrt-alphatest-no-buffer-zero-write:
https://patchwork.freedesktop.org/patch/118212

No regressions in Jenkins CI system.

Cc: "12.0 13.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>