gpu: ion: Add O_DSYNC flag to generated file

Ion uses the O_DSYNC flag of a file to indicate an uncached mapping.
When generating a file for mmaping, the new file needs to have the
same O_DSYNC flag as the parent (the file of the ioctl open call)
to indicate if the mapping should be cached or uncached.

Signed-off-by: Laura Abbott <lauraa@codeaurora.org>