drm/amdkfd: Don't assign dGPUs to APU topology devices

dGPUs need their own topology devices. Don't assign them to APU topology
devices with CPU cores.

Bug: https://github.com/RadeonOpenCompute/ROCK-Kernel-Driver/issues/66


Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Tested-by: Elias Konstantinidis <ekondis@gmail.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>