drm/meson: Fix invalid pointer in meson_drv_unbind()

[ Upstream commit 776e78677f514ecddd12dba48b9040958999bd5a ]

meson_drv_bind() registers a meson_drm struct as the device's privdata,
but meson_drv_unbind() tries to retrieve a drm_device. This may cause a
segfault on shutdown:

[ 5194.593429] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000197
 ...
[ 5194.788850] Call trace:
[ 5194.791349]  drm_dev_unregister+0x1c/0x118 [drm]
[ 5194.795848]  meson_drv_unbind+0x50/0x78 [meson_drm]

Retrieve the right pointer in meson_drv_unbind().

Fixes: bbbe775e ("drm: Add support for Amlogic Meson Graphic Controller")
Signed-off-by: NJean-Philippe Brucker <jean-philippe.brucker@arm.com>
Acked-by: NNeil Armstrong <narmstrong@baylibre.com>
Signed-off-by: NNeil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190322152657.13752-1-jean-philippe.brucker@arm.comSigned-off-by: NSasha Levin (Microsoft) <sashal@kernel.org>

drivers/gpu/drm/meson/meson_drv.c

@@ -317,8 +317,8 @@ static int meson_drv_bind(struct device *dev)
 
 static void meson_drv_unbind(struct device *dev)
 {
-	struct drm_device *drm = dev_get_drvdata(dev);
-	struct meson_drm *priv = drm->dev_private;
+	struct meson_drm *priv = dev_get_drvdata(dev);
+	struct drm_device *drm = priv->drm;
 
 	drm_dev_unregister(drm);
 	drm_kms_helper_poll_fini(drm);