drm/msm: add param to retrieve # of GPU faults (global)

For KHR_robustness, userspace wants to know two things, the count of GPU
faults globally, and the count of faults attributed to a given context.
This patch providees the former, and the next patch provides the latter.

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