mesa/st: Don't set alpha if ALPHA_TEST is lowered

When PIPE_CAP_ALPHA_TEST is zero, the driver does not support alpha testing, so alpha shouldn't be set. In particular, alpha.enable should be zero, since logically alpha testing is not used in the ZSA CSO when it's lowered in the fragment shader key.

Fixes failing asserts in kicad, rvgl, etc with Panfrost since 6afd4add. (We could remove the assert in panfrost instead, but logically setting alpha.enabled on top of lowering the shader seems wrong?)

Signed-off-by: Alyssa Rosenzweig alyssa.rosenzweig@collabora.com Reported-by: Icecream95 ixn@keemail.me Tested-by: Urja Rannikko urjaman@gmail.com Tested-by: Robin Murphy robin.murphy@arm.com Fixes: 6afd4add ("panfrost: Simplify depth/stencil/alpha")