drm/bridge: lt8912b: Add missing drm_bridge_attach call

The driver does not call drm_bridge_attach(), which causes the next
bridge to not be added to the bridge chain. This causes the pipeline
init to fail when DRM_BRIDGE_ATTACH_NO_CONNECTOR is used.

Add the call to drm_bridge_attach().

Fixes: 30e2ae94 ("drm/bridge: Introduce LT8912B DSI to HDMI bridge")
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230804-lt8912b-v1-4-c542692c6a2f@ideasonboard.com