drm: rcar-du: Don't create encoder for unconnected LVDS outputs

On R-Car D3 and E3, the LVDS encoders provide the pixel clock to the DU,
even when LVDS outputs are not used. For this reason, the rcar-lvds
driver probes successfully on those platforms even if no further bridge
or panel is connected to the LVDS output, in order to provide the
rcar_lvds_clk_enable() and rcar_lvds_clk_disable() functions to the DU
driver.

If an LVDS output isn't connected, trying to create a DRM connector for
the output will fail. Fix this by skipping connector creation in that
case, and also skip creation of the DRM encoder as there's no point in
an encoder without a connector.

Fixes: e9e05694 ("drm: rcar-du: lvds: Convert to DRM panel bridge helper")
Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
(cherry picked from commit 187502af)
Signed-off-by: Dave Airlie <airlied@redhat.com>