drm/amd/display: fix hotplug regression after code refactor

The condition logic of REG_WAIT in dce110_stream_encoder_dp_blank()
got inverted after refactoring.

Signed-off-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Jordan Lazare <Jordan.Lazare@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>