drm/loongson: Add platform dependency

Only offer the Loongson DRM driver as an option on platforms where
it makes sense.

Signed-off-by: Jean Delvare <[e-mail]>
Cc: Sui Jingfeng <[e-mail]>
Cc: David Airlie <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Reviewed-by: Sui Jingfeng <[e-mail]>
Signed-off-by: Sui Jingfeng <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/loongson/Kconfig

···
 config DRM_LOONGSON
 	tristate "DRM support for Loongson Graphics"
 	depends on DRM && PCI && MMU
+	depends on LOONGARCH || MIPS || COMPILE_TEST
 	select DRM_KMS_HELPER
 	select DRM_TTM
 	select I2C