MIPS: Loongson: Build ATI Radeon GPU driver as module

MIPS: Loongson: Build ATI Radeon GPU driver as module

When ATI Radeon GPU driver has been compiled directly into the kernel
instead of as a module, we should make sure the firmware for the model
(check available ones in /lib/firmware/radeon) is built-in to the kernel
as well, otherwise there exists the following fatal error during GPU init,
change CONFIG_DRM_RADEON=y to CONFIG_DRM_RADEON=m to fix it.

[    1.900997] [drm] Loading RS780 Microcode
[    1.905077] radeon 0000:01:05.0: Direct firmware load for radeon/RS780_pfp.bin failed with error -2
[    1.914140] r600_cp: Failed to load firmware "radeon/RS780_pfp.bin"
[    1.920405] [drm:r600_init] *ERROR* Failed to load firmware!
[    1.926069] radeon 0000:01:05.0: Fatal error during GPU init
[    1.931729] [drm] radeon: finishing device.

Fixes: 0d071c035ee0 ("MIPS: Loongson: Add a Loongson-3 default config file")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>