drm/amd/display: Re-enable Vsync Interrupts for Gradual Refresh Ramp

- Make sure Vsync interrupts are disabled in static screen case
  and enabled when not to save power
- Create no_static_for_external_dp debug option

Signed-off-by: Amy Zhang <Amy.Zhang@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>