msm: kgsl: Preemption switch to default pagetable if pagetable is destroyed

Pagetable could be detached and new set of commands to switch to
default pagetable could be submitted as part of default context.
But just before executing the commands to switch to default context
we could be preempted out of the ringbuffer. In the mean time old
pagetable could be destroyed. When we switch back to the ringbuffer
to execute commands to switch to default pagetable check if old
pagetable is destroyed and switch to default pagetable.

Change-Id: I09f569da7cb3a5601c49099c8122c2f1ff7adb65
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>