drm/amdgpu: get VCN start to process in the dpm disabled case

Fixes: 22cc6c5e (drm/amdgpu: Add runtime VCN PG support)

Signed-off-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>