msm: kgsl: Record the queued context timestamp in the snapshot

Record the last issued timestamp for each context in the snapshot.
With per context timestamps, each context has its own timestamp
sequence so it is important to know the last timestamp issued by
the context so we can reconstruct the failing frame in the snapshot
output.

Change-Id: Ic0dedbad9cd402aad4fb30a798a8ff5ab2a34950
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>