ARM: dts: cubieboard4: Remove the dumb-vga-dac compatible

The dumb-vga-dac and adi,adv7123 compatibles are not supposed to be used
together according to the binding.

Since the corpro gm7123 is a drop-in replacement for the adv7123, let's
remove dumb-vga-dac from our compatible list.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20210901091852.479202-47-maxime@cerno.tech