drm/i915: Refactor gmch hpd irq handling

Pull all the gmch platform hotplug interrupt handling into one
function.

v2: Move the IIR check to the caller
    s/drm_i915_private_t/struct drm_i915_private/

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Add posting read comment suggested by Chris.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>