drm/amdgpu: correct sdma7 max dw

correct sdma7 max dw into 8

Signed-off-by: Frank Min <Frank.Min@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>