drm/omap: Fix error handling path in 'omap_dmm_probe()'

If we don't find a matching device node, we must free the memory allocated
in 'omap_dmm' a few lines above.

Fixes: 7cb0d6c1 ("drm/omap: fix TILER on OMAP5")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>