[IMX8MM][MYON2] use video_mode_list to select display overlay

This MR fixes the second issue for reworked MR with "generic" support of MyonII in U-Boot.

The problem was described by SECO as follows:

[Michele] the displays list as not reason, as it is right now, as no overlay is selected; instead, the overlay is selected as a perihperal.

Please refer to !152 (comment 463054) for the full text.

As the selection of device tree overlay for display should be available via "seco_config video" and not via "seco_config peripheral":

• moved a definition of display from "overlay_peripheral_list" to "video_mode_list";
• the list of available displays has also been actualized.