drm/i915/cx0: prefer forward declarations over includes

Avoid including the world from headers when forward declarations
suffice.

Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230921155325.3851197-1-jani.nikula@intel.com