lima: implement alpha test

lima: implement alpha test

As [1] suggests, 3 lower bits in rsw->multi_sample is alpha test
compare func and bits 16-23 are reference value - uint8,
0.0 is 0, 0.5 is 0x80, 1.0 is 0xff.

Just as on Panfrost, enabled alpha test needs early-Z and pixel kill
to be disabled.

[1] http://web.archive.org/web/20171026123213/http://limadriver.org/Render_State/

Reviewed-by: Erico Nunes <nunes.erico@gmail.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@collabora.com>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10879>