video: bpp16: Disable by default on sunXi

The sunXi arm64 build has overflown, leading to the main U-boot binary
overwriting the environment when flashing the new image, or even worse,
overwriting itself when we're calling saveenv.

Disable this command that is not critical until we can adress the issue
properly.

Reviewed-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>