disp: msm: sde: modify setting of split_display flag

For CTL_ACTIVE targets, slave ctl need not to be reserved
as both the interfaces can be driven with single ctl.
Add a necessary check before enabling the feature.

Change-Id: Id68d7dd4fc1cf9534466fd5bfa50c3f551d06ce4
Signed-off-by: Jayaprakash <jmadiset@codeaurora.org>