drm/amd/display: Don't overwrite subvp pipe info in fast updates

[Description]
- This is a workaround to avoid concurrency issues -- a fast update
  creates a shallow copy of the dc current_state, and removes all
  subvp/phantom related flags.
- We want to prevent the fast update thread from removing those
  flags in case there's another thread running that requires
  the info for proper programming

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Jasdeep Dhillon <jdhillon@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>