drm/i915: constify the cdclk vtable

This is a bit of a twisty one since each platform is slightly
different, so might take some more review care.
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/75e0139b28cca9ceff77de72c3ef737c101255ba.1632869550.git.jani.nikula@intel.com

drivers/gpu/drm/i915/i915_drv.h

@@ -1006,7 +1006,7 @@ struct drm_i915_private {
 	const struct intel_audio_funcs *audio_funcs;
 
 	/* Display CDCLK functions */
-	struct intel_cdclk_funcs cdclk_funcs;
+	const struct intel_cdclk_funcs *cdclk_funcs;
 
 	/* PCH chipset type */
 	enum intel_pch pch_type;