drm/amdgpu: cleanup VM handling in the CS a bit

Add a helper function for getting the root PD addr and cleanup join the
two VM related functions and cleanup the function name.

No functional change.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>