gpu: ion: kmalloc_heap: flush cpu caches at allocation time

There's currently a problem with the kmalloc heap where buffers
occasionally come through unzero'd when mmap'd to userspace, which is a
security hole. We actually are zero'ing the buffer but that data isn't
immediately visible to folks with other mappings to the memory (e.g. the
mapping set up for userspace by the mmap handler). Fix this by flushing
the cpu caches at alloc time.

Change-Id: I3895c6bc163e2d065b48cae66e99825c5c0a865b
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>