videomodes: Add helper functions to parse video-mode env-var extra options

Add 2 helper functions to get strings, respectively integers from the options
value returned by video_get_video_mode() / video_get_ctfb_res_modes().

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Anatolij Gustschin <agust@denx.de>