drm/i915/display: move IPC under display wm sub-struct

Move display IPC related member under drm_i915_private display
sub-struct.

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/04ccaaceee9293e5a6c75761ba9d36792c36f095.1662983005.git.jani.nikula@intel.com