drm/tegra: Plug memory leak

Free the DRM device-private memory upon driver unload to make sure the
memory doesn't leak.
Signed-off-by: NThierry Reding <treding@nvidia.com>

drivers/gpu/drm/tegra/drm.c

@@ -118,6 +118,8 @@ static int tegra_drm_unload(struct drm_device *drm)
 		drm_mm_takedown(&tegra->mm);
 	}
 
+	kfree(tegra);
+
 	return 0;
 }