video: Fix kernel-doc warnings in of_display_timing + of_videomode

Fix kernel-doc warnings reported when using W=1.

v3:
  - Do not replace '-' with ':' (Thomas)

v2:
  - Improve subject (Lee)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: linux-fbdev@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20201128224114.1033617-2-sam@ravnborg.org