drm/i915: Split PORT_HOTPLUG_STAT ack out from i9xx_hpd_irq_handler()

drm/i915: Split PORT_HOTPLUG_STAT ack out from i9xx_hpd_irq_handler()

Split the VLV/CHV hoplug irq handling to ack and handler phases. This
way we can move the actual irq handling outside the section where
we have disabled the interrupt sources.

For now, we leave things as is for pre-VLV GMCH platforms, but
eventually they could get the same treatment.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460571598-24452-9-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>