drm/amd/display: Disable phantom OTG after enable for plane disable

[Description]
- Need to disable phantom OTG after it's enabled
  in order to restore it to it's original state.
- If it's enabled and then an MCLK switch comes in
  we may not prefetch the correct data since the phantom
  OTG could already be in the middle of the frame.

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>