drm/amdgpu: fix next_rptr handling for debugfs

BugLink: http://bugs.launchpad.net/bugs/1546572

That somehow got lost.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 41f2d990)
Signed-off-by: Alberto Milone <alberto.milone@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

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

@@ -487,7 +487,7 @@ static int amdgpu_debugfs_ring_info(struct seq_file *m, void *data)
 	seq_printf(m, "rptr: 0x%08x [%5d]\n",
 		   rptr, rptr);
 
-	rptr_next = ~0;
+	rptr_next = le32_to_cpu(*ring->next_rptr_cpu_addr);
 
 	seq_printf(m, "driver's copy of the wptr: 0x%08x [%5d]\n",
 		   ring->wptr, ring->wptr);