gma500: Fix Poulsbo suspend/resume crash on devices with SDVO ports

commit 7beff62e upstream.

Reported-by: Guillaume Clément <guillaume@baobob.org>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>