drm/syncobj: use dma_fence_get_stub

Extract of useful code from the timeline work. Let's use just a single
stub fence instance instead of allocating a new one all the time.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Link: https://patchwork.freedesktop.org/patch/265248/