msm: kgsl: Use the correct timestamp for the snapshot

The snapshot timestamp is currently based on wall time which makes
it very difficult to debug against a kernel log based on boot time.
It doesn't help matters that the snapshot timestamp is output in
human unfriendly hex.  Use boot time instead of wall time for the
snapshot value and print it out in decimal.

Change-Id: Ic0dedbadf9b7d799a3cb4d5917787bccd350c90a
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>