Revert "gpu: ion: system_heap: add support for 2MB allocations"

This reverts commit 561af5399e4ee229e74f7f3be042903063a10400.
Revert "ion: system_heap: only use low gfp flags for order-0 pages"

This reverts commit eeddd41b015fe729a82cb658c1321bc7807bebe8.