drm/amd/display: Don't lock phantom pipe on disabling

[Description]
- When disabling a phantom pipe, we first enable the phantom
  OTG so the double buffer update can successfully take place
- However, want to avoid locking the phantom otherwise setting
  DPG_EN=1 for the phantom pipe is blocked (without this we could
  hit underflow due to phantom HUBP being blanked by default)

Reviewed-by: Samson Tam <samson.tam@amd.com>
Acked-by: Stylon Wang <stylon.wang@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>