drm/modes: parse_cmdline: Rework drm_mode_parse_cmdline_options()

Refactor drm_mode_parse_cmdline_options() so that it takes a pointer
to the first option, rather then a pointer to the ',' before the first
option.

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/20191118155134.30468-5-hdegoede@redhat.com