drm/amdgpu: Disable DPC for XGMI for now.

XGMI support is more complicated than single device support as
questions of synchronization between the device recovering from
PCI error and other members of the hive are required.
Leaving this for next round.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>