msm: mdss: Check if secure display is enabled during non-secure validate

For video mode panels, when secure display has ended and non-secure
layers are being committed, check if secure display is still enabled.
Ideally there should be a NULL commit before non-secure content starts,
to clean up the secure display. If this isn't the case, add code to
detect, validate, and return failure.

Change-Id: I574547307073e38a19a42ff21679b17e1ec29b3b
Signed-off-by: Krishna Chaitanya Devarakonda <kdevarak@codeaurora.org>