drm/amdgpu/vcn: mmsch_v4_0_3 requires doorbell on 32 byte boundary

Align on 32 byte boundary.

Signed-off-by: Samir Dhume <samir.dhume@amd.com>
Acked-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>