drm/msm/a6xx: add module param to enable debugbus snapshot

For production devices, the debugbus sections will typically be fused
off and empty in the gpu device coredump.  But since this may contain
data like cache contents, don't capture it by default.

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