msm: kgsl: Change vma->vm_file to shmem file

If shmem is enabled, change vma->vm_file to shmem file. During mmap, the
vma is added to the vma chains contained in the adderss_space struct of
the file. This can later be used when trying to remove all mappings of
a page.

Change-Id: Ifa373c3300c4676fd2ba3d330d306060e4c321dc
Signed-off-by: Puranam V G Tejaswi <pvgtejas@codeaurora.org>