gpu: ion: Ensure zeroed memory gets written to RAM

When zeroing buffers, we need to make sure the zeroed data actually
gets written back to RAM. Otherwise, it may be possible for a buffer
to be reallocated with data still needing to be written back to RAM.
This may lead to corruption from userspace allocators expecting zero'd
buffers and a potential leak of kernel data to user space. Considering
the mapping is short lived and we need to write back the data, use a
writecombine mapping to write the data directly to ram and avoid the
cost of flushing the cache. If the mapping is cached, invalidate the
kernel side mappings as well.

Change-Id: I26a49908bde411469f3b8f143a1581fb4e2c3b56
CRs-Fixed: 520044
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>