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