gpu: ion: Fix several issues with page pool

Split out low and high mem pages so they are correctly reported
when the shrinker is called.
Fix potential deadlock caused by holding the page pool lock while
allocationg and also needing that lock from the shrink function

Change-Id: I6def3a3111f59914ef6b5b59d81a6fde37cb60c5
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>