drm/i915/gt: Ignore dt==0 for reporting underflows

The presumption was that some time would always elapse between recording
the start and the finish of a context switch. This turns out to be a
regular occurrence and emitting a debug statement superfluous.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201117113103.21480-4-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_lrc.c

@@ -1307,7 +1307,7 @@ static void reset_active(struct i915_request *rq,
 static void st_update_runtime_underflow(struct intel_context *ce, s32 dt)
 {
 #if IS_ENABLED(CONFIG_DRM_I915_SELFTEST)
-	ce->runtime.num_underflow += dt < 0;
+	ce->runtime.num_underflow++;
 	ce->runtime.max_underflow = max_t(u32, ce->runtime.max_underflow, -dt);
 #endif
 }
@@ -1324,7 +1324,7 @@ static void intel_context_update_runtime(struct intel_context *ce)
 	ce->runtime.last = intel_context_get_runtime(ce);
 	dt = ce->runtime.last - old;
 
-	if (unlikely(dt <= 0)) {
+	if (unlikely(dt < 0)) {
 		CE_TRACE(ce, "runtime underflow: last=%u, new=%u, delta=%d\n",
 			 old, ce->runtime.last, dt);
 		st_update_runtime_underflow(ce, dt);