drm/connector: Add documentation for drm_cmdline_mode

The struct drm_cmdline_mode holds the result of the command line parsers.
However, it wasn't documented so far, so let's do that.

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/963c893c16c6a25fc469b53c726f493d99bdc578.1560783090.git-series.maxime.ripard@bootlin.com