drm: Don't leak path blob property when updating

Previously, when updating the path blob property, we would leak the
existing one. Make this symmetrical with the tile and EDID blob
pointers.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Cc: Dave Airlie <airlied@redhat.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>