intel/fs: make alpha_to_coverage a tristate

That way in some cases we can do this dynamically.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21094>

src/intel/vulkan/anv_pipeline.c

@@ -546,7 +546,8 @@ populate_wm_prog_key(const struct anv_graphics_pipeline *pipeline,
     * code to workaround the issue that hardware disables alpha to coverage
     * when there is SampleMask output.
     */
-   key->alpha_to_coverage = ms != NULL && ms->alpha_to_coverage_enable;
+   key->alpha_to_coverage = ms != NULL && ms->alpha_to_coverage_enable ?
+      BRW_ALWAYS : BRW_NEVER;
 
    /* Vulkan doesn't support fixed-function alpha test */
    key->alpha_test_replicate_alpha = false;