ion: system_heap: only use low gfp flags for order-0 pages

By trying a little harder to get high-order pages (by virtue of the
liberal gfp flags used for high-order allocations) we can actually
optimize performance since high-order pages seem to help a lot. Only use
the low-order flags as a last resort, when trying to allocate an order-0
page.

Change-Id: If8d8f4b1b4503b0c7546cf5a4bcdcaecd2bb5905
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>