drm/i915: constify clock gating init vtable.

I used a macro to avoid making any really silly mistakes here.

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/7c97f7e5ab0eae3c4cd7ce8344254356c34f3ad6.1632869550.git.jani.nikula@intel.com