drm/i915/dsc: Add prefill helper for DSC

drm/i915/dsc: Add prefill helper for DSC

Add intel_vdsc_prefill_lines() which tells us how many extra lines
of latency the DSC adds to the pipe prefill.

We shouldn't need a "worst case" vs, "current case" split here
as the DSC state should only change during full modesets.

The returned numbers are in .16 binary fixed point.

Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20251014191808.12326-6-ville.syrjala@linux.intel.com