drm/i915/lrc: replace include with forward declarations

Prefer forward declarations over includes if possible.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220214173810.2108975-2-jani.nikula@intel.com
