drm/mgag200: Convert to simple KMS helper

The mgag200 supports a single pipeline with only a primary plane. It can
be converted to simple KMS helpers. This also adds support for atomic
modesetting. Wayland compositors, which use pageflip ioctls, can now be
used with mgag200.

v2:
	* prepare encoder and CRTC in a separate patch
	* remove suspend/resume code in a separate patch
	* don't call set_format_regs() in pipe_update()
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: John Donnelly <John.p.donnelly@oracle.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Emil Velikov <emil.velikov@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200515083233.32036-15-tzimmermann@suse.de

drivers/gpu/drm/mgag200/mgag200_drv.c

@@ -140,7 +140,7 @@ int mgag200_driver_dumb_create(struct drm_file *file,
 }
 
 static struct drm_driver driver = {
-	.driver_features = DRIVER_GEM | DRIVER_MODESET,
+	.driver_features = DRIVER_ATOMIC | DRIVER_GEM | DRIVER_MODESET,
 	.fops = &mgag200_driver_fops,
 	.name = DRIVER_NAME,
 	.desc = DRIVER_DESC,