[AGPGART] Rework AGPv3 modesetting fallback.

Sometimes the logic to handle AGPx8->AGPx4 fallback failed, as can
be seen in https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=197346

The failures occured if the bridge was in AGPx8 mode, but the
user hadn't specified a mode in their X config.  We weren't
setting the mode to the highest mode capable by the video card+bridge
(as we do in the AGPv2 case), which was leading to all kinds of
mayhem including us believing that after falling back from AGPx8, that
we couldn't do x4 mode (which is disastrous in AGPv3, as those are
the only two modes possible).

Signed-off-by: Dave Jones <davej@redhat.com>