dma-fence: Make ->enable_signaling optional

Many drivers have a trivial implementation for ->enable_signaling.
Let's make it optional by assuming that signalling is already
available when the callback isn't present.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180427061724.28497-4-daniel.vetter@ffwll.ch