msm: kgsl: Set the active context to null after MMU is stopped

This is needed to make sure that the last active pagetable
is still around until we are sure we no longer need the MMU.
This fixes a race condition between IFPC and pagetable destroy.
CP needs to save state to memory during IFPC and gets stuck
because the MMU has the stale pagetable.

Change-Id: I2338d3ad01fc7d8164ff7dc22ceea77ed8bc8354
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>
Signed-off-by: Kyle Piefer <kpiefer@codeaurora.org>