drm/i915/guc: Don't print out relay statistics when relay is disabled

If nobody has enabled the relay, we're not comunicating with GuC, which
means that the stats don't have any meaning. Let's also remove interrupt
counter and tidy the debugfs formatting.

v2: Correct stats accounting (Sagar)
v3: Corrected one more error in stats accounting, move relay_enabled (Sagar)

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180319095348.9716-9-michal.winiarski@intel.com