drm/amd/display: Don't check output BPP for phantom

drm/amd/display: Don't check output BPP for phantom

[Description]
- If we're enabling phantom pipe for a high
  link rate display we could
  fail DML on the phantom pipe since it's
  set to virtual signal
- Therefore don't consider Output BPP for
  phantom pipe

Reviewed-by: Dillon Varone <Dillon.Varone@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>