msm: kgsl: Mark mmapped objects with VM_DONTCOPY

Add VM_DONTCOPY to the default set of mmap flags to keep VM objects
from being copied on fork() and causing issues.  KGSL file descriptors
copied to a child are not expected to be usable.

Change-Id: Ic0dedbad85c07118a931ccb9f7a6fd0507da3e5a
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>