drm/i915: split out i915_list_util.h

Move list related utilities from i915_utils.h to separate new file
i915_list_util.h. Clean up related includes.

Note: Arguably none of this should exist in i915 in the first place. At
least isolate it better.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/d7526809735194137116682f37cfa126a6a87ec9.1757582214.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>