drm/msm: More fully implement devcoredump for a7xx

Use the vendor-provided snapshot headers to dump the contextless
registers, shader blocks, and cluster registers. Still unimplemented are
the GMU registers and "external core" registers, which would require
more work because they use register spaces we don't have described in
devicetree and dump registers from multiple spaces in a single list.
Signed-off-by: Connor Abbott <cwabbott0@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/575919/Signed-off-by: Rob Clark <robdclark@chromium.org>