drm/amdgpu: use cursor functions in amdgpu_bo_in_cpu_visible_vram

One of the last remaining uses of drm_mm_node.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Tested-by: Nirmoy Das <nirmoy.das@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>