drm: Fix compiler warning in drm_atomic_helper.c

Kbuild was complaining about:
>> drivers/gpu/drm/drm_atomic_helper.c:3169:27: warning: 'state' may be used uninitialized in this function [-Wmaybe-uninitialized]

Now state can't actually be used uninitialized, but we'll assign a value
anyways so it stops bellyaching.

Kbuild config:
	link:	https://lists.01.org/pipermail/kbuild-all/2018-November/055374.html
	tree:   git://anongit.freedesktop.org/drm/drm-misc for-linux-next
	head:   b7ea04d299
	commit: b7ea04d299 [4/4] drm: Add
	DRM_MODESET_LOCK_BEGIN/END helpers
	config: x86_64-randconfig-x017-201847 (attached as .config)
	compiler: gcc-7 (Debian 7.3.0-1) 7.3.0
	reproduce:
		git checkout b7ea04d299
		# save the attached .config to linux build tree
		make ARCH=x86_64

Fixes: b7ea04d299 ("drm: Add DRM_MODESET_LOCK_BEGIN/END helpers")
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
[seanpaul added extra details on airlied's suggestion]
Link: https://patchwork.freedesktop.org/patch/msgid/20181129203652.223634-1-sean@poorly.run

drivers/gpu/drm/drm_atomic_helper.c

@@ -3169,6 +3169,9 @@ struct drm_atomic_state *drm_atomic_helper_suspend(struct drm_device *dev)
 	struct drm_atomic_state *state;
 	int err;
 
+	/* This can never be returned, but it makes the compiler happy */
+	state = ERR_PTR(-EINVAL);
+
 	DRM_MODESET_LOCK_ALL_BEGIN(dev, ctx, 0, err);
 
 	state = drm_atomic_helper_duplicate_state(dev, &ctx);