drm/i915/debugfs: Add i915_hpd_storm_ctl

This adds a file in i915's debugfs directory that allows userspace to
manually control HPD storm detection. This is mainly for hotplugging
tests, where we might want to test HPD storm functionality or disable
storm detection to speed up hotplugging tests without breaking anything.

Changes since v1:
- Make HPD storm interval configurable
- Misc code cleanup

Signed-off-by: Lyude <lyude@redhat.com>
Acked-by: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Tomeu Vizoso <tomeu@tomeuvizoso.net>