ion: Don't create a kernel mapping in begin_cpu_access

Currently ION buffer kernel mappings are being created during
begin_cpu_access and are removed during end_cpu_access.

The begin_cpu_access and end_cpu_access calls don't need a kernel mapping
and creating kernel mappings unnecessarily for large buffers can be
expensive because of the alloc_vmap_area call.

Change ION so that kernel mappings are only created when they are needed,
which is for calls such as dma_buf_kmap and dma_buf_vmap.

Change-Id: I60b06455bedab1a7bfab83eb6013475c8ae638b5
Signed-off-by: Liam Mark <lmark@codeaurora.org>