drm/amdgpu: add Raven chip id case for ucode

Set the appropriate ucode loading mechanism.  Set to
direct for now.

Signed-off-by: Chunming Zhou <David1.Zhou@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>