drm/omap: panel-dpi: make (limited) use of a reset gpio

Some displays have a reset input. To assert that the display is
functional the reset gpio must be deasserted.

Teach the driver to get and drive such a gpio accordingly.

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