drm/i915: Handle -EDEADLK in drm_atomic_commit from load-detect.

CI runs with DEBUG_WW_MUTEX_SLOWPATH, so -EDEADLK occurs a lot more.
Handle the case where drm_atomic_commit fails with -EDEADLK correctly.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/56D3FEF1.6070306@linux.intel.com
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -10577,7 +10577,8 @@ found:
 		goto fail;
 	}
 
-	if (drm_atomic_commit(state)) {
+	ret = drm_atomic_commit(state);
+	if (ret) {
 		DRM_DEBUG_KMS("failed to set mode on load-detect pipe\n");
 		goto fail;
 	}