drm: export atomic wait_for_vblanks helper (v2)

v1: original
v2: danvet's kerneldoc nitpicks

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>