drm/amd/display: Update V_UPDATE whenever VSTARTUP changes

[Why]
If VSTARTUP changes due to bandwidth requirements, we must
recalculate and update VLINE2 as well for proper flip reporting.

[How]
After all calls to program_global_sync which reconfigures
VSTARTUP, also make sure to update V_UPDATE (i.e. VLINE2 on DCNx).

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Sivapiriyan Kumarasamy <Sivapiriyan.Kumarasamy@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>