i965: Set prog_data->uses_kill if simulating alpha test via discards.

i965: Set prog_data->uses_kill if simulating alpha test via discards.

When using MRT on Gen4-5, we have to simulate GL's alpha test feature
by emitting discards in the fragment shader.  In this case, it makes
sense to set prog_data->uses_kill, which means the fragment shader may
kill pixels via the discard mechanism.

This saves us from having to look an extra key value in a couple of
places, including in the generator.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>