MIPS: loongson64: Fix no screen display during boot-up

The Framebuffer CONFIG_FB needs to be explicitly selected or we
don't get any framebuffer anymore. DRM has stopped to select FB
after commit f611b1e7

 ("drm: Avoid circular
dependencies for CONFIG_FB") because of circular dependency. So
we should enable it in the default config file, otherwise there
is no display before Xorg.

Signed-off-by: Wang Haojun <wanghaojun@loongson.cn>
Reviewed-by: Huacai Chen <chenhuacai@kernel.org>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>