drm: Fix locking gotcha in page_flip ioctl

We want to lock the primary plane, not the cursor (which might be
optional). Real bad case of copy-paste fail, unfortunately our CI
didn't catch that because i915 does have a cursor plane.

Reported-by: Eric Anholt <eric@anholt.net>
Fixes: 29dc0d1 ("drm: Roll out acquire context for the page_flip ioctl")
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170330204831.8225-1-daniel.vetter@ffwll.ch
Tested-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>

drivers/gpu/drm/drm_plane.c

@@ -872,7 +872,7 @@ int drm_mode_page_flip_ioctl(struct drm_device *dev,
 	ret = drm_modeset_lock(&crtc->mutex, &ctx);
 	if (ret)
 		goto out;
-	ret = drm_modeset_lock(&crtc->cursor->mutex, &ctx);
+	ret = drm_modeset_lock(&crtc->primary->mutex, &ctx);
 	if (ret)
 		goto out;