gpu: ion: Change ION_IOC_ALLOC ioctl return value

ION_IOC_ALLOC may incorrectly return an error pointer
which could be 0 when an allocation fails. The ioctl call from user
space expects that the function succeeded when ioctl returns 0.
Change ION_IOC_ALLOC return value to -ENOMEM when the requested
buffer could not be allocated.

Change-Id: Id9d49ffa0eb8af5defc9635f40b45351b9dd8999
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>