msm: kgsl: Defer the pagetable destroy to a separate thread

The pagetable destroy process requires a mutex, so it must be called
from a context that can sleep. Defer it to a thread that can sleep.

Change-Id: I0e86e0f88223715ac4bb7b8085707d135e79379d
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>