drm/i915/cdclk: Add prefill helpers for CDCLK

Add helpers to compute the CDCLKl adjustment factor for prefill
calculations. The adjustment factor is always <= 1.0. That is,
a faster CDCLK speeds up the pipe prefill.

intel_cdclk_prefill_adjustment_worst() gives out a worst case estimate,
meant to be used during guardband sizing. We can actually do better
than 1.0 here because the absolute minimum CDCLK is limited by the
dotclock. This will still allow planes, pfit, etc. to be changed any
which way without having to resize the guardband yet again.

intel_cdclk_prefill_adjustment() is supposed to give a more accurate
value based on the current min cdclk for the pipe, but currently that
is not yet available when this gets called. So for now use the same
worst case estimate here.

The returned numbers are in .16 binary fixed point.

TODO: the intel_cdclk_prefill_adjustment_worst() approach here
      can result in guardband changes for DRRS. But I'm thinking
      that is fine since M/N changes will always happen on the
      legacy timing generator so guardband doesn't actually matter.
      May need to think about this a bit more though...

v2: Use the worst case estimate always for now

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/20251014191808.12326-4-ville.syrjala@linux.intel.com

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