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 <[e-mail]>
Tested-by: Mikhail Gavrilov <[e-mail]>
Reviewed-by: Christian König <[e-mail]>
Signed-off-by: Bert Karwatzki <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c

···
 		 * next job actually sees the results from the previous one
 		 * before we start executing on the same scheduler ring.
 		 */
-		if (!s_fence || s_fence->sched != sched)
+		if (!s_fence || s_fence->sched != sched) {
+			dma_fence_put(fence);
 			continue;
+		}
 
 		r = amdgpu_sync_fence(&p->gang_leader->explicit_sync, fence);
+		dma_fence_put(fence);
 		if (r)
 			return r;
 	}