drm/i915: abstract hpd irq storm detection

Simplify intel_hpd_irq_handler() by extracting HPD irq storm detection
to a separate function.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>