drm/amdgpu: switch firmware path for SI parts

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

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>