gpu: ion: Use writecombine for uncached mappings

When we map ion buffers as uncached we want to map them as
uncached but still be able to use buffering of transactions
since turning off buffering has a severe performance impact.

Change-Id: I5a09ac34bd3ce3f001d10d9efd71d140f632564d
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>