msm: kgsl: Disable all yield packets for secure contexts

Preemption of secure context is not supported in A5x. Currently we
disable preemptive context switching during execution of commands from
secure contexts by placing appropriate PREEMPT_ENABLE_GLOBAL/LOCAL
packets in ringbuffer. These packets have no effect on the behavior of
CONTEXT_SWITCH_YIELD packet. So a cooperative context switch (yield) can
still be serviced. To avoid this, disable all yield packets in case of
secure contexts.

Change-Id: Icfd73795ca4dccfc04f7a5b4497a908b15794e5a
Signed-off-by: Puranam V G Tejaswi <pvgtejas@codeaurora.org>