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.

Change-Id: I6d338d7dca680f57f2c39ed5a14381b261bf8bce
Signed-off-by: Anshuman Dani <adani@codeaurora.org>