MIPS: Loongson64: loongson3_defconfig: Enable amdgpu drm driver

As it's usuable on LS3A4000 platform. Tested with RX550, glmark2
got about 4235 score.

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