drm: sde: Select correct path for split display

When the split display has preferred control path, the preferred
control path ID should be cleared before searching secondary control
path.  Otherwise the search would fail.

Change-Id: Ie2cc1c4d0ec2d162cdcfedbbbef62400e5798bb6
Signed-off-by: Camus Wong <camusw@codeaurora.org>