drm/amd/display: Fix Subvp phantom pipe transition

[Description]
- When transitioning a pipe in use (non-phantom) to become
  a phantom pipe, we must fully disable the pipe first before
  doing any phantom pipe programming
- Move phantom OTG enablement to after "regular" front-end
  programming sequence (including wait for vupdate)
- If a pipe is being transitioned into a phantom pipe,
  fully disable it first

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@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>