drm/gma500: Use simple encoder

The gma500 driver uses empty implementations for some of its encoders.
Replace the code with the generic simple encoder. As a side effect, the
patch also removes an indirection in the encoder setup for Medfield.

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: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200305155950.2705-6-tzimmermann@suse.de