drm: rcar-du: Fix NULL encoder pointer dereference

The DRM connector's encoder pointer is managed internally by the DRM
core and set to NULL when the DRM connector is disconnected from the
CRTC it was attached to. This results in a NULL pointer dereference in
the HDMI connector functions when trying to call the associated slave
encoder's operations.

Fix this by retrieving the slave encoder pointer from the R-Car
connector structure instead of the DRM connector structure.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>