videomodes: Add video_get_ctfb_res_modes helper function

Add a video_get_ctfb_res_modes() helper function, which uses
video_get_video_mode() to parse the 'video-mode' environment variable and then
looks up the matching mode in res_mode_init and returns the matching mode.

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