drm/amdgpu/dce11: optimize pageflip

Taking the grph update lock is only necessary when
updating the the secondary address (for single pipe stereo).

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