drm/omap: dss: Remove unused omapdss_default_get_timings()

All omap_dss_driver instances provide the get_timings operation. Remove
the default function.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>