Mips: loongson3_defconfig: Enable ast drm driver by default

ASpeed GPU is typically found on the Loongson server platform, as a
peripheral device driver, it generally should be compiled as a module.

Tested on loongson 3B4000 server[1].

[1] https://github.com/loongson-gfx/loongson_boards/tree/main/ls3b4000x2_server



Signed-off-by: Sui Jingfeng <suijingfeng@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>