msm: mdss: release secure UI buffer as soon as pipe is destroyed

When secure UI session is in progress, need to make sure that the
buffers are released as soon as pipe is freed to make sure secure
session can be properly terminated without any secure UI buffers
remaining as mapped.

CRs-Fixed: 619802
Change-Id: Ia2fa87f3182efc170304a53eb8d6b3f614355245
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>