drm/amd/display: Fix 300Hz Freesync bug

Needed to reprogram vblank_start in dml properly in order to get the
correct dlg params to program VTG.

Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>