drm/amdgpu/si_dma: remove unused variable in si_dma_stop()

After commit 571c0536 ("drm/amdgpu: switch sdma buffer function
tear down to a helper"), the variable 'ring' is not used anymore, it
can be removed.
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

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

@@ -112,14 +112,12 @@ static void si_dma_ring_emit_fence(struct amdgpu_ring *ring, u64 addr, u64 seq,
 
 static void si_dma_stop(struct amdgpu_device *adev)
 {
-	struct amdgpu_ring *ring;
 	u32 rb_cntl;
 	unsigned i;
 
 	amdgpu_sdma_unset_buffer_funcs_helper(adev);
 
 	for (i = 0; i < adev->sdma.num_instances; i++) {
-		ring = &adev->sdma.instance[i].ring;
 		/* dma0 */
 		rb_cntl = RREG32(DMA_RB_CNTL + sdma_offsets[i]);
 		rb_cntl &= ~DMA_RB_ENABLE;