drm/radeon: fix count in cik_sdma_ring_test()

Should be 5 rather than 4.

Noticed-by: Mathias Fröhlich <Mathias.Froehlich@gmx.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Christian König <christian.koenig@amd.com>