intel/fs: Set src0 alpha present bit in header when provided in message payload.

Currently the "Source0 Alpha Present to RenderTarget" bit of the RT
write message header is derived from brw_wm_prog_data::replicate_alpha.
However the src0_alpha payload is provided anytime it's specified to
the logical message.  This could theoretically lead to an
inconsistency if somebody provided a src0_alpha value while
brw_wm_prog_data::replicate_alpha was false, as I'm planning to do in
a future commit in order to implement a hardware workaround.

Instead calculate the header bit based on whether a src0_alpha value
was provided to the logical message, which guarantees the same
behavior on pre-ICL and ICL+ (the latter used an extended descriptor
bit for this which didn't suffer from the same issue).  Remove the
brw_wm_prog_data::replicate_alpha flag.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/intel/compiler/brw_fs_visitor.cpp

@@ -493,7 +493,7 @@ fs_visitor::emit_fb_writes()
     * so we compute if we need replicate alpha and emit alpha to coverage
     * workaround here.
     */
-   prog_data->replicate_alpha = key->alpha_test_replicate_alpha ||
+   const bool replicate_alpha = key->alpha_test_replicate_alpha ||
       (key->nr_color_regions > 1 && key->alpha_to_coverage &&
        (sample_mask.file == BAD_FILE || devinfo->gen == 6));
 
@@ -506,7 +506,7 @@ fs_visitor::emit_fb_writes()
          ralloc_asprintf(this->mem_ctx, "FB write target %d", target));
 
       fs_reg src0_alpha;
-      if (devinfo->gen >= 6 && prog_data->replicate_alpha && target != 0)
+      if (devinfo->gen >= 6 && replicate_alpha && target != 0)
          src0_alpha = offset(outputs[0], bld, 3);
 
       inst = emit_single_fb_write(abld, this->outputs[target],