drm/i915/cdclk: Add intel_cdclk_min_cdclk_for_prefill()

drm/i915/cdclk: Add intel_cdclk_min_cdclk_for_prefill()

Introduce a helper to compute the min required cdclk frequency
for a given guardband size. This could be used to bump up the
cdclk in case the vblank is so small that the normally computed
minimum cdclk results in too slow a prefill.

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/20251014191808.12326-5-ville.syrjala@linux.intel.com