gpu: ion: Add explicit sync ioctl

This is deprecated in favor of using the dma_buf api which will
automatically sync a buffer to memory when it is mapped to a device.
However, that functionality is not ready, so this patch adds the
ability to sync a buffer explicitly.

Change-Id: Ia15810a13cd5c4b939f4afa5c8e721c89fac76d4
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>