drm/radeon/dce4+: optimize pageflip (v2)

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

v2: fix comment as per Michel's suggestion

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>