msm: kgsl: Initialize cb_num for secure context

Currently we are not initializing cb_num for secure context.
This is causing problems in the kgsl fault handler path when
there is a pagefault for secure context. Hence initialize cb_num
for secure context.

Change-Id: I35a37193f512f315f443e9f0d92fdae11411bbd2
Signed-off-by: Harshitha Sai Neelati <hsaineel@codeaurora.org>