gpu: ion: Fix incorrect argument to map call

An incorrect argument is passed to a local ion heap
function call. This is causing incorrect flags to be
passed to iommu map call. The side effect is that the
flags passed to the iommu map call is defaulted to
read-write and uncached.

Change-Id: Ia2a7e64818dc8f251dadfb545f9b218a5e3cfc42
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>