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

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

Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://lore.kernel.org/r/61ae2013c5db962e90e072be7d37d630cb7dfc34.1744119460.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

Author: jnikula