drm/amdgpu: fix check for port PM availability

The ATPX method does not always exist on the dGPU, it may be located at
the iGPU. The parent device of the iGPU is the root port for which
bridge_d3 is false. This accidentally enables the legacy PM method which
conflicts with port PM and prevented the dGPU from powering on.

Fixes: 1db4496f ("drm/amdgpu: fix power state when port pm is unavailable")

Reported-and-tested-by: Mike Lothian <mike@fireburn.co.uk>
Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: <stable@vger.kernel.org> # 4.8+