video: mmp: drop needless devm cleanup

The nice thing about devm_* is that the driver doesn't need to free the
resources but the driver core takes care about that. This also
simplifies the error path quite a bit and removes the wrong check for a
clock pointer being NULL.

Russell King - ARM Linux <linux@arm.linux.org.uk>:
"And this patch also fixes the above: disabling/unpreparing _after_ putting
the thing - which was quite silly... :)"

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>