OMAPDSS: VENC: Check for errors from regulator_enable()

It is possible for regulator_enable() to fail and if it does fail that's
generally a bad sign for anything we try to do with the hardware afterwards
so check for and immediately return an error if regulator_enable() fails.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>