modesetting: Fix inverted check in dri2 WaitMSC

ms_queue_vblank() returns false on failure.

Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Tested-by: Mike Lothian <mike@fireburn.co.uk>