gpu: ion: Change from uncached to buffered mapping

PMEM uses buffered mappings when PMEM device is
opened with O_SYNC flag. ION should have the same
behavior as PMEM.

Change ION to provide buffered mappings instead
of uncached when the O_DSYNC flag is used when
opening the ION device.

CRs-fixed: 335827
Change-Id: I369a35a2ff68adc8339380dfbb7721e3d361ee69
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>