drm/amdgpu: Disable RAS by default on APU flatform

Disable RAS feature by default for aqua vanjaram on APU platform.

Changed from V1:
	Splite Disable RAS by default on APU platform into a
	separated patch.

Changed from V2:
	Avoid to modify global variable amdgpu_ras_enable.

Signed-off-by: Stanley.Yang <Stanley.Yang@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>