drm/vc4: Use simple encoder

The vc4 driver uses empty implementations for its encoders. Replace
the code with the generic simple encoder.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200305155950.2705-19-tzimmermann@suse.de