drm/{i915, xe}/display: Add display runtime pm parent interface

drm/{i915, xe}/display: Add display runtime pm parent interface

We have differing implementations for display runtime pm in i915 and xe
drivers. Add struct of function pointers into display_parent_interface
which will contain used implementation of runtime pm.

v2:
  - add _interface suffix to rpm function pointer struct
  - add struct ref_tracker forward declaration
  - use kernel-doc comments

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patch.msgid.link/20251030202836.1815680-3-jouni.hogander@intel.com