gpu: ion: Do fallback when allocating large sizes

The preferred allocation of memory for kernel data structures in
ION is to use kmalloc. This is mainly for performance reasons. However,
we are not guaranteed to be able to allocate large sizes. So to keep
the code performing well and at the same time allow for larger allocations,
implement a fallback mechanism for allocation of memory for ION kernel data
structures.

Change-Id: I8c6a13ad74cce69f590920d28c01a58f7a540fd2
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>