OMAPDSS: panel-dpi: enable-gpio

The enable gpio should be optional, but the driver returns an error if
it doesn't get the gpio.

So change the driver to accept -ENOENT error.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Tested-by: Joachim Eastwood <manabian@gmail.com>