msm: kgsl: Add postmortem dump for 2D (z180) cores

Add ringbuffer and register dump as part of postmortem dump for z180
cores. Also, add kgsl_postmortem_dump as a preparatory general function
for postmortem dump for both types of kgsl devices, adreno and z180.

Change-Id: I8b538771bfa8f6bfdfe0b1b993afa3c53f8eb8cf
Signed-off-by: Harsh Vardhan Dwivedi <hdwivedi@codeaurora.org>