drm: rcar-du: Use drmm_encoder_alloc() to manage encoder

The encoder allocation was converted to a DRM managed resource at the
same time as the addition of a new helper drmm_encoder_alloc() which
simplifies the same process.

Convert the custom drm managed resource allocation of the encoder
with the helper to simplify the implementation, and prevent hitting a
WARN_ON() due to the handling the drm_encoder_init() call directly
without registering a .destroy() function op.

Fixes: f5f16725 ("drm: rcar-du: Use DRM-managed allocation for encoders")
Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>