drm: rcar-du: lvds: Fix LVDS startup on R-Car Gen3

According to the latest revisions of the R-Car Gen3 manual, the LVDS mode
must be set before the LVDS I/O pins are enabled, not after -- fix the
Gen3 LVDS startup sequence accordingly.

Fixes: e947eccb ("drm: rcar-du: Add support for LVDS mode selection")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
[Updated comment in rcar_du_lvdsenc_start_gen3()]
[Moved Gen2 startup comment update to separate commit]
[Fixed =| typo]
Tested-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>