drm/i915/wm: convert i9xx_wm.c to intel_de_*() register interface

The registers handled in i9xx_wm.c are mostly display registers. The
MCH_SSKPD and MLTR_ILK registers are not. Convert register access to
intel_de_*() interface where applicaple.

Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://lore.kernel.org/r/68367382759570413669d5648895a1da8f6c68f7.1744119460.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

Author: jnikula