Revert "drm/amdgpu: don't use ATRM for external devices"

This reverts commit 8d4237a1 which is
commit 432e664e upstream.

It breaks the current Android ABI, and if needed, can be brought back in
an abi-safe way in the future.

Bug: 161946584
Change-Id: I4465f1886d5a4a886ac4ba6de4e77f14de20a145
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>