gpu: ion: Add cache maintenance to ion.

This patch adds cache maintenance operations to ion.  As per mailing
list discussions regarding dma_buf, cache operations are done implicitly.
At buffer allocaiton time the user can select whether he'd like mappings
(both kernel and user) to be cached.  When cached mappings are selected,
no mappings will be created for a buffer at mmap time.  Instead pages will
be faulted in one at a time so we can track which pages require flushing
before dma.  When the buffers are mapped for dma (via the dma_buf apis)
any pages which were touched will be synced for device.

Change-Id: Id5d6894e8bb52af038c91dd895143bf3b4203b0b
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>