msm: mdss: update secure display validation checks

Non-secure pipe should not be staged on MDSS when
secure display session is in progress. Client has
to unstage all pipes to disable the secure session
before pushing non-secure layers on MDP. This change
updates the validation checks to avoid such conditions.

Change-Id: I626c6c3a5ef313b1af8369884a9b10d8586f7251
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>