drm: Fix wrong kfree() in managed resource usage example

The example code showing how to use the managed resource API calls
kfree() on the wrong pointer. Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200402095325.5266-1-laurent.pinchart+renesas@ideasonboard.com