drm/amdgpu: correct emit frame size for vcn dec/enc ring

only mmhub will be invalidated during vcn dec/enc vm flush

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