drm/syncobj: Rename fence_get to find_fence

The function has far more in common with drm_syncobj_find than with
any in the get/put functions.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Christian König <christian.koenig@amd.com> (v1)
Signed-off-by: Dave Airlie <airlied@redhat.com>