Add the const that was accidentally left out from the vtables.

Fixes: 6b4cd9cb ("drm/i915: constify the cdclk vtable")
Cc: Dave Airlie <airlied@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211021133408.32166-1-jani.nikula@intel.com
(cherry picked from commit 877d0749)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>