gma500: opregion and ACPI

Add the opregion support and bring us in line with the opregion functionality in the
reference driver code. We can't share this with i915 currently because there are
hardcoded assumptions about dev_priv etc in both versions.

[airlied: include opregion.h fix]

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>