drm/msm/a6xx: Cleanup indexed regs const'ness

These tables were made non-const in commit 3cba4a2c ("drm/msm/a6xx:
Update ROQ size in coredump") in order to avoid powering up the GPU when
reading back a devcoredump.  Instead let's just stash the count that is
potentially read from hw in struct a6xx_gpu_state_obj, and make the
tables const again.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/592699/