drm/doc: Fix missing kerneldoc for drm_dp_helper.c

Never added when the DP validation code was added.

Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468612088-9721-5-git-send-email-daniel.vetter@ffwll.ch