drm/i915/gen4: Extra CRT hotplug paranoia

Disable the CRT plug interrupt while doing the force cycle, explicitly
clear any CRT interrupt we may have generated, and restore when done.
Should mitigate interrupt storms from hotplug detection.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Eric Anholt <eric@anholt.net>