drm/i915/cdclk: Introduce intel_cdclk_modeset_checks()

drm/i915/cdclk: Introduce intel_cdclk_modeset_checks()

I plan to better decouple the cdclk computation from actual
modesets. To that end make the cdclk code self sufficient in
being able to determine if a full cdclk calculation/update is
needed or not due to some not strictly cdclk related reason.

Currently we have three such things that depend active_pipes:
- cdclk_state->actual
- glk audio w/a
- dg2 power well stuff

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250923171943.7319-8-ville.syrjala@linux.intel.com
Reviewed-by: Mika Kahola <mika.kahola@intel.com>