drm/i915: move hotplug_funcs to display.funcs

Move display hotplug functions under drm_i915_private display
sub-struct.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1cf677f1d02b8b621ec41ff77fe5de26d4f6954a.1661346845.git.jani.nikula@intel.com