gpu: ion: ion_system_heap: Change allocations to GFP_HIGHUSER

Previously the ion_system_heap was using GFP_KERNEL, forcing all allocations
to be in lowmem.  This quickly causes us to run out of lowmem.

Change-Id: I6d502a5f1c20c8a8e44fb5903fcb228f8435a564
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>