drm/i915: Debugfs interface for GuC submission statistics

This provides a means of reading status and counts relating
to GuC actions and submissions.

v2:
    Remove surplus blank line in output [Chris Wilson]

v5:
    Added GuC per-engine submission & seqno statistics

v6:
    Add per-ring statistics to client, refactor client-dumper.

Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Alex Dai <yu.dai@intel.com>
Reviewed-by: Tom O'Rourke <Tom.O'Rourke@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>