msm: kgsl: Restrict secure contexts to ringbuffer level preemption

Preemption from secure to unsecure needs zap shader to be run
to clear all secure content. CP does not know during preemption
if it is switching between secure and unsecure contexts so
restrict secure contexts to be preempted at ringbuffer level.
At the end of each secure submission we switch back to unsecure
mode and run the zap shader to clear secure contents. Ringbuffer
level preemption ensures Zap shader is run before switching
back to unsecure mode.

CRs-Fixed: 974102
Change-Id: Iff11c1d5732d46fe5a1fbdbc7d162aaa1736741b
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>