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

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