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

After commit 571c05365892 ("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: Yang Yingliang <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

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

···
 
 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;