Use separate firmware path for amdgpu to avoid conflicts
with radeon on CIK parts.

v2: squash in logic simplification (Alex)
Reviewed-by: NChunming Zhou <david1.zhou@amd.com>
Reviewed-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>