drm/amdgpu: fix sdma v4 ring is disabled accidently

For sdma v4, there is bug caused by
commit d4e869b6b5d6 ("drm/amdgpu: add ring test for page queue")'

local variable ring is reused and changed, so amdgpu_ttm_set_buffer_funcs_status(adev, true)
is skipped accidently. As a result, amdgpu_fill_buffer() will fail, kernel message:

[drm:amdgpu_fill_buffer [amdgpu]] *ERROR* Trying to clear memory with ring turned off.
[   25.260444] [drm:amdgpu_fill_buffer [amdgpu]] *ERROR* Trying to clear memory with ring turned off.
[   25.260627] [drm:amdgpu_fill_buffer [amdgpu]] *ERROR* Trying to clear memory with ring turned off.
[   25.290119] [drm:amdgpu_fill_buffer [amdgpu]] *ERROR* Trying to clear memory with ring turned off.
[   25.290370] [drm:amdgpu_fill_buffer [amdgpu]] *ERROR* Trying to clear memory with ring turned off.
[   25.319971] [drm:amdgpu_fill_buffer [amdgpu]] *ERROR* Trying to clear memory with ring turned off.
[   25.320486] amdgpu 0000:19:00.0: [mmhub] VMC page fault (src_id:0 ring:154 vmid:8 pasid:32768, for process  pid 0 thread  pid 0)
[   25.320533] amdgpu 0000:19:00.0:   in page starting at address 0x0000000000000000 from 18
[   25.320563] amdgpu 0000:19:00.0: VM_L2_PROTECTION_FAULT_STATUS:0x00800134
Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c

@@ -1151,10 +1151,11 @@ static int sdma_v4_0_start(struct amdgpu_device *adev)
 		}
 
 		if (adev->sdma.has_page_queue) {
-			ring = &adev->sdma.instance[i].page;
-			r = amdgpu_ring_test_ring(ring);
+			struct amdgpu_ring *page = &adev->sdma.instance[i].page;
+
+			r = amdgpu_ring_test_ring(page);
 			if (r) {
-				ring->ready = false;
+				page->ready = false;
 				return r;
 			}
 		}