msm: kgsl: Keep postamble packets in a privileged buffer

Postamble packets are executed in privileged mode by gpu. So we should keep
them in a privileged scratch buffer to block userspace access. for
targets with APRIV feature support, we can mark the preemption scratch
buffer as privileged too to avoid similar issues in future.

Change-Id: Ifda360dda251083f38dfde80ce1b5dc83daae902
Signed-off-by: Akhil P Oommen <[email protected]>
Signed-off-by: Kaushal Sanadhya <[email protected]>