gpu: ion: repurpose flags field for heap_mask and caching

In the latest Ion API, userspace clients specify the heap_mask for an
allocation in the `heap_mask' field of the ion_allocation_data
structure, and they specify caching in the `flags' field. Before, the
heap_mask was in the `flags' field and caching was specified with the
O_DSYNC file open flag.

We would like to support the new API without actually updating Ion. To
do this, | the heap_mask back into the flags field on an allocation
ioctl (since the internal kernel APIs still expect to see it
there). Also make sure that caching is taken from the buffer flags
field rather than the O_DSYNC file flag.

Change-Id: I872afb40d9bf7c49bb31f19c5e9726ac714ef850
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>