drm/amdgpu: Use the ALIGN() macro

Use the ALIGN() macro to set "num_dw" to a
multiple of 8, i.e. lower 3 bits cleared.

Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>