drm: Fix comment on mode parsing

Do not claim that there's a default mode in the video= option parser.
if no option string has been given, the parser does nothing.

Signed-off-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: Javier Martinez Canillas <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_modes.c

···
  * @mode: preallocated drm_cmdline_mode structure to fill out
  *
  * This parses @mode_option command line modeline for modes and options to
- * configure the connector. If @mode_option is NULL the default command line
- * modeline in fb_mode_option will be parsed instead.
+ * configure the connector.
  *
  * This uses the same parameters as the fb modedb.c, except for an extra
  * force-enable, force-enable-digital and force-disable bit at the end::