board: ge: bx50v3: use VPD instead of compile-time checks

B{46}50v3s have an internal LCD that needs to be configured,
in comparison with B850v3 which has only external displays.

Use VPD instead of `CONFIG_TARGET_GE_B{4,6,8}50V3' compile-time
checks to correct initialize video based on the monitor type.

Signed-off-by: Ian Ray <ian.ray@ge.com>
Signed-off-by: Nandor Han <nandor.han@ge.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>