gma500: Only register interrupt handler for poulsbo hardware

First step in adding proper irq handling. We'll start with poulsbo support so
make sure other chips don't touch drm_irq_install().

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>