i965/gs: Check newly-generated GS-out VUE map against correct stage

i965/gs: Check newly-generated GS-out VUE map against correct stage

Previously, we compared our new GS-out VUE map to the existing *VS*-out
VUE map, which is bogus.

This would mostly manifest as redundant dirty flagging where the GS is
in use but the VS and GS output layouts differ; but there is a scary
case where we would fail to flag a GS-out layout change if it happened
to match the VS-out layout.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Cc: "10.5, 10.4" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=88885