drm/amd/display: only polling VSync Phase within VSync peroroid

Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>