drm/i915/display: Use display parent interface for i915 runtime pm

drm/i915/display: Use display parent interface for i915 runtime pm

Start using display parent interface for i915 runtime pm. Doing the same
for xe is done in coming changes.

v3:
  - remove useless include
v2:
  - use <> when including drm/intel/display_parent_interface.h
  - drop checks for validity of rpm function pointers

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patch.msgid.link/20251030202836.1815680-6-jouni.hogander@intel.com