drm/amd/display: Only update FP2 for full updates

[Why]
FP2 is not double buffered and must wait for VACTIVE
before programming.

[How]
Only update when there is a full update we should
change FP2 to avoid delay every flip.

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>