drm/msm/a6xx: Fix a6xx indexed-regs in devcoreduump

Somehow, possibly as a result of rebase gone badly, setting
nr_indexed_regs for pre-a650 a6xx devices lost the setting of
nr_indexed_regs, resulting in values getting snapshot, but omitted
from the devcoredump.

Fixes: e997ae5f ("drm/msm/a6xx: Mostly implement A7xx gpu_state")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/640289/