drm/msm/a6xx: Add a6xx gpu state

Add support for gathering and dumping the a6xx GPU state including
registers, GMU registers, indexed registers, shader blocks,
context clusters and debugbus.

v2: Fix bugs discovered by Sharat Masetty

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>