drm/amdgpu: remove unused code

Remove the unused function - amdgpu_vm_pt_is_root_clean
and remove the impossible condition

v1: entries == 0 is not possible any more,
    so this condition could probably be removed (Felix)

Signed-off-by: Jesse Zhang <jesse.zhang@amd.com>
Suggested-by：Felix Kuehling <felix.kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>