glsl: don't try to lower non-gl builtins as if they were gl_FragData

glsl: don't try to lower non-gl builtins as if they were gl_FragData

If a shader has an output array, it will get treated as though it were
gl_FragData and rewritten into gl_out_FragData instances. We only want
this to happen on the actual gl_FragData and not everything else.

This is a small part of the problem pointed out by the below bug.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96765
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: "11.2 12.0" <mesa-stable@lists.freedesktop.org>