glsl: Tidy stream handling in merge_qualifier().

glsl: Tidy stream handling in merge_qualifier().

The previous commit fixed xfb_buffer handling, which was largely copy
and pasted from the stream handling.  The difference is that stream
was set in input_layout_mask, so it worked.

However, that's totally rubbish: stream is only valid on geometry shader
outputs.  Presumably this was to hack around inout.  Instead, apply the
solution I used in the previous fix.

Really, we just need to separate shader interface and parameter
qualifier handling so this isn't a mess, but this patch at least
tidies it slightly.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>