drm/i915: Split GEM object type definition to its own header

For convenience in avoiding inline spaghetti, keep the type definition
as a separate header.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190528092956.14910-1-chris@chris-wilson.co.uk