drm/radeon: fix typo in cik_copy_dma

Otherwise we end up with a rather strange looking result.

Signed-off-by: Christian König <christian.koenig@amd.com>
Tested-by: Tom Stellard <thomas.stellard@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>