drm/i915/wm: convert i9xx_wm.c internally to struct intel_display

Going forward, struct intel_display is the main display device data
pointer. Convert as much as possible of i9xx_wm.c to struct
intel_display.

Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://lore.kernel.org/r/bbee93f837fe7fedfd1627ff6fa295da8881df8d.1744119460.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

Author: jnikula