drm/amdgpu: fix no sync_wait in copy_buffer

when eviction is happening, if don't handle
dependency, then the fence could be dead off.

Signed-off-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>
Reviewed-by: Christian K?nig <christian.koenig@amd.com>