drm/i915/pmdemand: convert to struct intel_display

Going forward, struct intel_display is the main display device
structure. Convert pmdemand to it.

Cc: Gustavo Sousa <gustavo.sousa@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/c1d92e9490013d5aba50fc1d1ebc0ee18e82cf7e.1735662324.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>