msm: kgsl: Add CP_WAIT_FOR_IDLE packet before updating timestamp

HW workaround to resolve MMU pagefault caused by memory
freeing early before GPU completes it.

This is race condition where timestamp is getting updated
and the event handler runs before the GPU actually got done
with the timestamp, hence freeing the buffer early while GPU
still not completed with it.

Signed-off-by: Anshuman Dani <adani@codeaurora.org>
(cherry picked from commit faf69cfeeede0883b9556e532d720ab4dc95b01d)

Change-Id: I6105250faff58da588e4bb6b3f4101e49e5c4758
Signed-off-by: Gopal Goberu <ggober@codeaurora.org>