drm/amdgpu: fix memory leak in amdgpu_cs_sync_rings

amdgpu_sync_get_fence deletes the returned fence from the
syncobj, so the refcount of fence needs to lowered to avoid
a memory leak.

Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/2360


Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Bert Karwatzki <spasswolf@web.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/3b590ba0f11d24b8c6c39c3d38250129c1116af4.camel@web.de