Move display hotplug related members under drm_i915_private display
sub-struct.

Rename struct i915_hotplug to intel_hotplug while at it.
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/c1c7562a31c115e9d6a131861e4ca9c97d4f7e09.1661346845.git.jani.nikula@intel.com