drm/amd/display: Blank phantom OTG before enabling

[Description]
Before enabling the phantom OTG for an update we
must enable DPG to avoid underflow.

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>