msm: kgsl: Use dma_buf_get() to get dma_buf structure

Currently we don't ensure if vma->vm_file is associated with dma_buf. This
can cause issues later when private_data from a non dma_buf file is used as
dma_buf structure. Hence get the fd that is associated with vma->vm_file
and use dma_buf_get() to get pointer to dma_buf structure. dma_buf_get()
ensures that the file from the input fd is associated with dma_buf.

Change-Id: Ib78aef8b16bedca5ca86d3a132278ff9f07dce73
Signed-off-by: Puranam V G Tejaswi <quic_pvgtejas@quicinc.com>