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