drm/bridge: Add interlace_allowed flag to drm_bridge

In preparation for a connector creation helper based on a chain of
bridges, add a flag to the drm_bridge structure to report support for
interlaced modes. This will be used to set the connector's
interlace_allowed flag.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200226112514.12455-9-laurent.pinchart@ideasonboard.com