drm/i915/wm: Add WM0 prefill helpers

drm/i915/wm: Add WM0 prefill helpers

Add skl_wm0_prefill_lines() (based on the actual state) and
skl_wm0_prefill_lines_worst() (worst case estimate) which
tell us how many extra lines are needed in prefill for WM0.

The returned numbers are in .16 binary fixed point.

TODO: skl_wm0_prefill_lines_worst() is a bit rough still

v2: Drop all pre-icl FIXMEs since this only gets used for VRR guardband

Reviewed-by: Uma Shankar <uma.shankar@intel.com> #v1
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20251014191808.12326-8-ville.syrjala@linux.intel.com