gpu: ion: Fix lockdep issue in ion_page_pool

Currently the mutex is held while kmalloc is called, under a low memory
condition this might trigger the shrinker which also takes this mutex.
Refactor so the mutex is not held during allocation.

Change-Id: Ic1d3b2f69e61209191bac84724ba56f6b98e2bc4
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>