drm/ast: Use simple encoder

The ast driver uses an empty implementation for its encoder. Replace
the code with the generic simple encoder.

v2:
	* rebase onto new simple-encoder interface

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200228081828.18463-3-tzimmermann@suse.de