i965/fs: Fix slight layering violation in emit_single_fb_writes().

i965/fs: Fix slight layering violation in emit_single_fb_writes().

In cases where the color0 argument wasn't being provided,
emit_single_fb_writes() would take the alpha channel directly from the
visitor state instead of taking it from its arguments.  This sort of
hack didn't fit nicely into the logical send-message approach because
all parameters of the instruction have to be visible to the SIMD
lowering pass for it to be able to split them into halves at all.

Fix it by using LOAD_PAYLOAD in fs_visitor::emit_fb_writes() to
provide an actual color0 vector with undefined contents except for the
alpha component to match the previous behavior when no color buffers
are enabled.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>