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_compiler.h

@@ -751,7 +751,6 @@ struct brw_wm_prog_data {
    bool dispatch_16;
    bool dispatch_32;
    bool dual_src_blend;
-   bool replicate_alpha;
    bool persample_dispatch;
    bool uses_pos_offset;
    bool uses_omask;