msm: mdss: Add support to get/set secure flag via an ioctl

Add support to get/set secure flag for WB session through an ioctl.
This is required for WB clients such as AD/WFD, enabling them to modify
or query the secure flag depending on requirements e.g. secure video
playback when using AD.

Change-Id: I852b6f79b31b5ba027d9a5c11561c562023cb99f
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>