drm/mgag200: Replace simple-KMS with regular atomic helpers

Drop simple-KMS in favor of regular atomic helpers. Makes the code
more modular and hence better to adapt to per-model requirements.

The simple-KMS helpers provide few extra features, so the patch is
mostly about open-coding what simple-KMS does. The simple-KMS helpers
do mix up plane and CRTC state. Changing to regular atomic helpers
requires to split some of the simple-pipe functions into per-plane
and per-CRTC code

No functional changes.

v3:
	* always run drm_atomic_helper_check_plane_state()
	* clean up style

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220728124103.30159-8-tzimmermann@suse.de