drm/i915/cdclk: Document CDCLK update methods

Add a bit of documentation to briefly explain the methods
by which we can change the CDCLK frequency.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240207013334.29606-5-ville.syrjala@linux.intel.com


Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>