drm/syncobj: Fix oops on drm_syncobj_find_fence(file_priv, 0, ...).

drm/syncobj: Fix oops on drm_syncobj_find_fence(file_priv, 0, ...).

This broke rendering on V3D, where we almost always have a 0
in-syncobj.

Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: 48197bc564c7 ("drm: add syncobj timeline support v9")
Cc: Chunming Zhou <david1.zhou@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.kernel.org/patch/10669317/
Signed-off-by: Christian König <christian.koenig@amd.com>