msm: kgsl: Read the A6XX register crashdumper data properly

Save the crashdumper offset so that we can read the registers from
the correct place in the crashdumper data.

Change-Id: I94303aee9250563c48bb5c816e259538bff06af0
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>