ANDROID: staging: ion: implement vmap for built-in heaps

dma_buf_vmap() on ION buffers returns -EOPNOTSUPP
if the heap of ION buffer does not implement its own
vmap callback. Even the default system heap does not
implement vmap callback, so vmap attempt of ION buffer
will always fail. However vmap of ION buffer is much
more reasonable to be supported by default than errors,
so this patch implements vmap callback.

Bug: 150258007

Signed-off-by: Hyesoo Yu <hyesoo.yu@samsung.com>
Change-Id: I243eb7ded49843e719532ad1b1cb7ab37e84460d