drm/amd/display: delay fp2 programming until vactive before lock

[Why]
race condition of programming FP2 wrt pipe locking
and vactive state can cause underflow/black screen

[How]
Enforce the FP2 is only programmed during vactive,
and unlock pipe soon afterwards.

Signed-off-by: Martin Leung <martin.leung@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>