drm/atomic: Initialize variables in drm_atomic_helper_async_check() to make gcc happy

drm_atomic_helper_async_check() declares the plane, old_plane_state and
new_plane_state variables to iterate over all planes of the atomic
state and make sure only one plane is enabled.

Unfortunately gcc is not smart enough to figure out that the check on
n_planes is enough to guarantee that plane, new_plane_state and
old_plane_state are initialized.

Explicitly initialize those variables to NULL to make gcc happy.

Fixes: fef9df8b ("drm/atomic: initial support for asynchronous plane update")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180724133300.32023-1-boris.brezillon@bootlin.com

drivers/gpu/drm/drm_atomic_helper.c

@@ -1510,8 +1510,9 @@ int drm_atomic_helper_async_check(struct drm_device *dev,
 {
 	struct drm_crtc *crtc;
 	struct drm_crtc_state *crtc_state;
-	struct drm_plane *plane;
-	struct drm_plane_state *old_plane_state, *new_plane_state;
+	struct drm_plane *plane = NULL;
+	struct drm_plane_state *old_plane_state = NULL;
+	struct drm_plane_state *new_plane_state = NULL;
 	const struct drm_plane_helper_funcs *funcs;
 	int i, n_planes = 0;