drm: rcar-du: lvds: Fix crash in .atomic_check when disabling connector

The connector .atomic_check() handler can be called with a NULL crtc
pointer in the connector state when the connector gets disabled
explicitly (through performing a legacy mode set or setting the
connector's CRTC_ID property to 0). This causes a crash as the crtc
pointer is dereferenced without any check.

Fix it by returning from the .atomic_check() handler when then crtc
pointer is NULL, as there is no check to be performed when the connector
gets disabled.

Fixes: c6a27fa4

 ("drm: rcar-du: Convert LVDS encoder code to bridge driver")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>