drm/i915: factor out valleyview_pipestat_irq_handler

This will be used by other platforms too, so factor it out.

The only functional change is the reordeing of gmbus_irq_handler() wrt.
the hotplug handling, but since it only schedules a work, it isn't an
issue.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: Don't keep on using the private_t typedef.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>