drm/amd/display: enable DPG when disabling plane for phantom pipe

[Why]
In disable_dangling_plane, for phantom pipes, we enable OTG so
disable programming gets the double buffer update.  But this
causes an underflow to occur.

[How]
Enable DPG prior to enabling OTG.

Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Samson Tam <samson.tam@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>