dri3: Don't fail on version mismatch

dri3: Don't fail on version mismatch

The previous commit to make DRI3 modifier support optional, breaks with
an updated server and old client.

Make sure we never set multibuffers_available unless we also support it
locally. Make sure we don't call stubs of new-DRI3 functions (or empty
branches) which will never succeed.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Fixes: 7aeef2d4efdc ("dri3: allow building against older xcb (v3)")