drm/mgag200: Use simple encoder

The mgag200 driver uses an empty implementation for its encoder. Replace
the code with the generic simple encoder.

v4:
	* print error message with drm_err()
v3:
	* init pre-allocated encoder with drm_simple_encoder_init()
v2:
	* rebase onto new simple-encoder interface

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200228081828.18463-4-tzimmermann@suse.de