drm/amdgpu: Add amdgpu_atpx_get_dhandle()

Since it seems that some vendors are storing the ATIF ACPI methods under
the same handle that ATPX lives under instead of the device's own
handle, we're going to need to be able to retrieve this handle later so
we can probe for ATIF there.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org