drm/i915: Handle error paths during watermark sanitization properly (v3)

sanitize_watermarks() does not properly handle errors returned by
drm_atomic_helper_duplicate_state().  Make failures drop locks before
returning.  We also change the lock of connection_mutex to a
drm_modeset_lock_all_ctx() to make sure any EDEADLK's are handled
earlier.

v2: Change call to lock connetion_mutex with a call to
    drm_modeset_lock_all_ctx().  This ensures that any lock contention
    is handled earlier and drm_atomic_helper_duplicate_state() won't
    return EDEADLK. (Maarten)

v3: Drop locks properly in more error paths. (Maarten)

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1452611617-32144-1-git-send-email-matthew.d.roper@intel.com

drivers/gpu/drm/i915/intel_display.c

@@ -15355,17 +15355,17 @@ static void sanitize_watermarks(struct drm_device *dev)
 	 */
 	drm_modeset_acquire_init(&ctx, 0);
 retry:
-	ret = drm_modeset_lock(&dev->mode_config.connection_mutex, &ctx);
+	ret = drm_modeset_lock_all_ctx(dev, &ctx);
 	if (ret == -EDEADLK) {
 		drm_modeset_backoff(&ctx);
 		goto retry;
 	} else if (WARN_ON(ret)) {
-		return;
+		goto fail;
 	}
 
 	state = drm_atomic_helper_duplicate_state(dev, &ctx);
 	if (WARN_ON(IS_ERR(state)))
-		return;
+		goto fail;
 
 	/*
 	 * Hardware readout is the only time we don't want to calculate
@@ -15388,7 +15388,7 @@ retry:
 		 * BIOS-programmed watermarks untouched and hope for the best.
 		 */
 		WARN(true, "Could not determine valid watermarks for inherited state\n");
-		return;
+		goto fail;
 	}
 
 	/* Write calculated watermark values back */
@@ -15401,6 +15401,7 @@ retry:
 	}
 
 	drm_atomic_state_free(state);
+fail:
 	drm_modeset_drop_locks(&ctx);
 	drm_modeset_acquire_fini(&ctx);
 }