radeonsi: pass alpha_ref value to PS in the user sgpr

radeonsi: pass alpha_ref value to PS in the user sgpr

Currently it's hardcoded in the shader, so every change requires
compilation of the shader variant, killing the performance
in Serious Sam 3 and probably other apps.

This patch passes alpha_ref in the user sgpr and removes it from
the shader key.

Signed-off-by: Vadim Girlin <vadimgirlin@gmail.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>