drm/i915/display: convert power domain code internally to struct intel_display

Going forward, struct intel_display is the main device data structure
for display. Convert intel_display_power.c internally first, leaving
external interfaces for follow-up.

v2: Rebase, checkpatch fixes

Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/d3284b30b53dd2fec786775ccb8992939360d774.1732808222.git.jani.nikula@intel.com