i965: Always emit alpha when nr_color_buffers == 0.

i965: Always emit alpha when nr_color_buffers == 0.

If alpha-testing is enabled, we need to send alpha down the pipeline
even if nr_color_buffers == 0.  However, tracking whether alpha-testing
is enabled in the WM program key is expensive: it causes us to compile
multiple specializations of the same shader, using program cache space.

This patch removes the check for alpha-testing, and simply emits alpha
whenever nr_color_buffers == 0.  We believe this will also be necessary
for alpha-to-coverage, and it should add minimal overhead to an uncommon
case.  Saving the recompiles should more than make up the difference.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>