drm/amd/display: Return last used DRR VTOTAL from DC

[How]
Add call to get the last used VTOTAL from DC

Signed-off-by: Jayendran Ramani <Jayendran.Ramani@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>