drm/vblank: Remove old-style comments

Somehow I forgot a few when typing all the shiny new kerneldoc. Drop
them.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181005073636.27291-1-daniel.vetter@ffwll.ch