i965/msaa: Set KILL_ENABLE when GL_ALPHA_TO_COVERAGE enabled.

i965/msaa: Set KILL_ENABLE when GL_ALPHA_TO_COVERAGE enabled.

i965 hardware needs to be informed of situations in which it's
possible for pixels (or samples) to be discarded for reasons other
than depth/stencil testing (e.g. due to an explicit "discard" in the
fragment shader).  One of these situations is when
GL_ALPHA_TO_COVERAGE is enabled, since that can cause samples to be
discarded by the color calculator when the pixel's alpha value is less
than 1.0.

Without this patch, GL_ALPHA_TO_COVERAGE does not take effect on depth
buffers.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>