drm/amd/display: Retain phantom pipes when min transition into subvp (#7358)

[Description]
- When entering into a SubVP config that requires a minimal
  transition we need to retain phantom pipes and also restore
  the mall config
- This is because the min transition will remove phantom pipes
  from the context (shallow copy) and not restore it's original
  state
- This is just a workaround, and needs a proper fix

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>