drm/atomic: Print debug message on atomic check failure

Does what it says on the label, it's a little confusing debugging atomic
check failures otherwise.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NLyude Paul <lyude@redhat.com>
Reviewed-by: NDhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180411234302.2896-2-lyude@redhat.com

drivers/gpu/drm/drm_atomic.c

@@ -1702,11 +1702,15 @@ int drm_atomic_check_only(struct drm_atomic_state *state)
 		}
 	}
 
-	if (config->funcs->atomic_check)
+	if (config->funcs->atomic_check) {
 		ret = config->funcs->atomic_check(state->dev, state);
 
-	if (ret)
-		return ret;
+		if (ret) {
+			DRM_DEBUG_ATOMIC("atomic driver check for %p failed: %d\n",
+					 state, ret);
+			return ret;
+		}
+	}
 
 	if (!state->allow_modeset) {
 		for_each_new_crtc_in_state(state, crtc, crtc_state, i) {