drm/amd/display: Save addr update in scratch before flip

[Why & How]
Fix a minor sequencing issue where the address update for
a subvp flip should be saved in scratch registers before
the actual flip

Reviewed-by: Austin Zheng <austin.zheng@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>