drm/omap: Expose DRM modes instead of timings in display devices

omap_dss_device operations expose fixed video timings through a
.get_timings() operation that return a single timing for the device. To
prepare for the move to drm_bridge, modify the API to instead add DRM
modes directly to the connector.

As this puts more burden on display devices, we also create a helper
function for panels to add a single DRM mode from the panel video
timings.

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