drm/i915: move and group cdclk under display.cdclk

Move display cdclk related members under drm_i915_private display
sub-struct.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7df23655be5dc70fb1a2b43ce41e1682e40395d8.1661779055.git.jani.nikula@intel.com