drm/doc: Add kerneldoc for @index

Was forgotten when adding them all over. 0-day should complain about
new missing kernel-doc, not sure why that wasn't caught/fixed.

v2: Clarify that @index is invariant, as discussed with Chris Wilson.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468612088-9721-2-git-send-email-daniel.vetter@ffwll.ch