drm/i915: Add kerneldoc to describe i915_gem_object.vma_list

drm/i915: Add kerneldoc to describe i915_gem_object.vma_list

Add kerneldoc for the vma_list stored on the i915_gem_object, in
particular, documenting the expected ordering of elements -- i.e. that
we do expect GGTT VMA first followed by the ppGTT VMA.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170525204818.12044-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>