msm: kgsl: Clean up error paths in KGSL/Adreno snapshot

We make our best effort in the GPU snapshot code but errors are not
fatal. We need to recover cleanly from errors but we don't need to
pass them around. Clean up and simplify the return paths.

Change-Id: Ic0dedbad7d3228aac428d7755e2e4b929f684ace
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>