drm/i915: use runtime irq suspend/resume in freeze/thaw

We don't need to uninstall the full handler, simply disabling interrupts
ought to be enough.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>