drm/i915/irq: abstract irq storm hotplug disabling

Continue abstracting hotplug storm related functions to clarify the
code. This time, abstract hotplug irq storm related hotplug
disabling. While at it, clean up the loop iterating over connectors for
readability.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>