drm/atomic-helper: properly annotate functions in kerneldoc

Without the () the markup and more important hyperlinking wont happen.

v2: Also fix nearby type Laurent spotted.

v3: Actually git add. Argh!

Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>