drm/amd/display: Disconnect phantom pipe OPP from OPTC being disabled

[Why]
If an OPP is used for a different OPTC without first being disconnected
from the previous OPTC, unexpected behaviour can occur. This also
applies to phantom pipes, which is what the current logic missed.

[How]
Disconnect OPPs from OPTC for phantom pipes before disabling OTG master.

Also move the disconnection to before the OTG master disable, since the
register is double buffered.

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: George Shen <george.shen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>