drm/i915/hotplug: move hotplug storm debugfs to intel_hotplug.c

The debugfs should be where the implementation details are.

v2: Rebase

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220916130634.3781122-1-jani.nikula@intel.com