gpu: ion: Modify gfp flags in ion_system_heap

When allocations larger than order 4 are made, use _GFP_NORETRY
and __GFP_NO_KSWAPD so kswapd doesn't get kicked off to reclaim
these larger chunks.  For smaller allocaitons, these are
unnecessary, as the system should be able to reclaim these.

Change-Id: I6b45cd1a595d25cc39622e3dbcc04d84fa1b7db8
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>