drm/msm: Fix debugbus snapshot

We weren't setting the # of captured debugbus blocks.

Reported-by: Connor Abbott <cwabbott0@gmail.com>
Suggested-by: Connor Abbott <cwabbott0@gmail.com>
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/666660/