drm/i915/display: update to plane_wm register access function

drm/i915/display: update to plane_wm register access function

Future platforms can have new additions in the plane_wm
registers. So update skl_wm_level_from_reg_val() to have
possiblity for such platform differentiations. This is in
preparation for the rest of the patches in this series where
hw support for the minimum and interim ddb allocations for
async flip is added. Replace all the i915 uses to intel_display
in this function while updating this function

Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241121112726.510220-2-vinod.govindapillai@intel.com