drm: restore open_count if drm_setup fails

If drm_setup (called at first open) fails, the whole
open call has failed, so we should not keep the
open_count incremented.

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Cc: stable@vger.kernel.org
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>