drm/panel: Add and initialise an orientation field to drm_panel

Current usage of drm_connector_set_panel_orientation is from a panel's
get_modes call. However if the panel orientation property doesn't
exist on the connector at this point, then drm_mode_object triggers
WARNs as the connector is already registered.

Add an orientation variable to struct drm_panel and initialise it from
drm_panel_init.
panel_bridge_attach can then create the property before the connector
is registered.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>