msm: kgsl: Fix race condition in snapshot sysfs read

If there are concurrent sysfs reads of snapshot binary
there can be a race condition where the snapshot data
is prematurely free'd by one reader while the other reader
is still reading it. Fix this by proper refcounting using
an atomic.

CRs-Fixed: 902816
Change-Id: I7a156c3a22f5475df0394ae30328d0fd6140f3da
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>