sunxi: video: Add extra modes and allow selecting the mode via hdmi_mode env

Add the following extra modes:

1280x720@50
1920x1080@60
1920x1200@60

And allow selecting them by setting (and then saving and rebooting) a
hdmi_mode env variable to the name of the mode.

Also make the reserved fb mem slightly larger to allow 1920x1200 to work.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>