[U-Boot] CONFIG_DISPLAY causes build error on microblaze-generic

[Bin Meng]

Hi Simon, Anatolij,

At present CONFIG_DISPLAY is set to y by default when driver model is
on. This causes the following building error on microblaze-generic.

  drivers/built-in.o: In function 'display_read_timing':
  drivers/video/display-uclass.c:45: undefined reference to 'edid_get_timing'

I think we should make CONFIG_DISPLAY 'default n' and update whatever
boards who want DM video support to explicitly turn in on in its board
defconfig files. This will fix microblaze-generic build error too.

Regards,
Bin