msm: mdss: fix secure session validation

During secure to non secure transition the commit contains
only non-secure layers and there is no null commit in between.
This change correctly validates such commit and allows non secure layers
only and the driver then performs the functionality to transition to
non-secure state.

CRs-Fixed: 1110585
Change-Id: Ic1d32f0d04d8ee860844f59fd99707dbdb78838f
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>