Revert "drm/amdgpu: simplify ATPX detection"

This reverts commit f5fda6d8.

You can't use BASE_CLASS in pci_get_class.

Bug: https://gitlab.freedesktop.org/drm/amd/issues/995


Acked-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>