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 autotest needs early-Z and pixel kill to be disabled.
[1] http://web.archive.org/web/20171026123213/http://limadriver.org/Render_State/
Signed-off-by: Vasily Khoruzhick anarsoul@gmail.com