Add a freestanding function argument to drm_mode_parse_cmdline_options()
similar to how drm_mode_parse_cmdline_extra() already has this.

This is a preparation patch for allowing parsing of stand-alone options
without a mode before them, e.g.: video=HDMI-1:margin_right=14,...
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191118155134.30468-6-hdegoede@redhat.com