drm/amdgpu: Add a read after write DB_CTRL for vcn_v4_0_3

To make sure VCN DB_CTRL is delivered before doorbell write.

Signed-off-by: Sonny Jiang <sonny.jiang@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>