drm/i915: make i915_utils.h self-contained

And ensure it stays that way.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/69bcebefa6d8689d4a962394b0c6db04904354ed.1556809195.git.jani.nikula@intel.com