m68k: defconfig: Disable fbdev on Sun3/3x

Commit e019630e ("[PATCH] remove broken video drivers") in
v2.6.21 removed Sun3 framebuffer support, hence there are no more frame
buffer device drivers available on the Sun3 and Sun3x platforms.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20220207131123.2203141-1-geert@linux-m68k.org