drm/amd/display: Multi-display underflow observed

[Why]
FP2 programming not happening when topology changes occur with multiple
displays.

[How]
Ensure FP2 is programmed whenever global sync changes occur but wait for
VACTIVE first to avoid underflow.

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Acked-by: Bindu Ramamurthy <bindu.r@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>