drm/radeon/kms: r600/r700 disable irq at suspend

To avoid hw doing anythings after we disabled PCIE GART, fully
disable IRQ at suspend. Also cleanup a bit the ih structure
and process function.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@linux.ie>