drm/gpuvm: fall back to drm_exec_lock_obj()

Fall back to drm_exec_lock_obj() if num_fences is zero for the
drm_gpuvm_prepare_* function family.

Otherwise dma_resv_reserve_fences() would actually allocate slots even
though num_fences is zero.

Cc: Christian König <christian.koenig@amd.com>
Acked-by: Donald Robson <donald.robson@imgtec.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231129220835.297885-2-dakr@redhat.com