msm: mdss: add proper layer zorder validation

Current validate logic expects proper ordering of layers in the
validate list, however there is no proper validation if this is not
the case. The following 2 scenarios are not caught:
* If there are more than 1 layer in the same z-order and the
  subsequent layers are not directly to the right of previous one,
  then the previous layer is replaced (i.e. removed) without
  notification to user. Yielding to unexpected result.
* If the list is not ordered, there is no validation whether there is
  any other layer already using same z-order.

Add code to validate these uncaught scenarios.

CRs-Fixed: 987777
Change-Id: Ie6e6bfdaf1cdc6a3f408fdb0a0227607f5ff7ac1
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>