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

drivers/gpu/drm/mgag200/mgag200_drv.h

@@ -15,11 +15,13 @@
 
 #include <video/vga.h>
 
+#include <drm/drm_connector.h>
+#include <drm/drm_crtc.h>
 #include <drm/drm_encoder.h>
 #include <drm/drm_fb_helper.h>
 #include <drm/drm_gem.h>
 #include <drm/drm_gem_shmem_helper.h>
-#include <drm/drm_simple_kms_helper.h>
+#include <drm/drm_plane.h>
 
 #include "mgag200_reg.h"
 
@@ -276,9 +278,11 @@ struct mga_device {
 	enum mga_type			type;
 
 	struct mgag200_pll pixpll;
+	struct drm_plane primary_plane;
+	struct drm_crtc crtc;
+	struct drm_encoder encoder;
 	struct mga_i2c_chan i2c;
 	struct drm_connector connector;
-	struct drm_simple_display_pipe display_pipe;
 };
 
 static inline struct mga_device *to_mga_device(struct drm_device *dev)