drm/msm/sde: fix validation in secure state check

Invalid secure mode check in planes, during atomic check
in crtc leads to failures during secure playback cases.
Add proper validations to avoid it.

Change-Id: I0b5096959c6166f413aa905077de60d42f43c516
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>