drm: Add kerneldoc markup for new @scan parameters in drm_mm

A couple of parameters slipped through the kerneldoc net.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161228105120.14500-1-chris@chris-wilson.co.uk