gma500: Don't enable MSI on Poulsbo

Chipset reports MSI capabilities for Poulsbo even though it isn't really there.

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>