drm/amdgpu: don't put the root PD into the relocated list

Instead of skipping the root PD while processing the relocated list just never
put it on the list in the first place.

This avoids walking the list all together when the root PD is the only entry
and so also avoids trying to submit a zero sized IB to the SDMA.

Signed-off-by: Christian König <christian.koenig@amd.com>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>