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: I3ebbc3e1e618aeb2b974bcc032e89f097eb526d5
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>
Git-commit: 1fc3f5102601e392d644762e2f79e774c1e225e3
Git-repo: https://android.googlesource.com/kernel/common


Signed-off-by: Laura Abbott <lauraa@codeaurora.org>