drm/simple-helper: drm_gem_simple_display_pipe_prepare_fb as default

It's tedious to review this all the time, and my audit showed that
arcpgu actually forgot to set this.

Make this the default and stop worrying.

Again I sprinkled WARN_ON_ONCE on top to make sure we don't have
strange combinations of hooks: cleanup_fb without prepare_fb doesn't
make sense, and since simpler drivers are all new they better be GEM
based drivers.

v2: Warn and bail when it's _not_ a GEM driver (Noralf)

v3: It's neither ... nor, not not (Sam)

Acked-by: Sam Ravnborg <[e-mail]>
Cc: Sam Ravnborg <[e-mail]>
Cc: Noralf Trønnes <[e-mail]>
Acked-by: Noralf Trønnes <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Cc: Maarten Lankhorst <[e-mail]>
Cc: Maxime Ripard <[e-mail]>
Cc: Thomas Zimmermann <[e-mail]>
Cc: David Airlie <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_simple_kms_helper.c

···
 #include <drm/drm_atomic.h>
 #include <drm/drm_atomic_helper.h>
 #include <drm/drm_bridge.h>
+#include <drm/drm_drv.h>
+#include <drm/drm_gem_atomic_helper.h>
 #include <drm/drm_managed.h>
 #include <drm/drm_plane_helper.h>
 #include <drm/drm_probe_helper.h>
···
 	struct drm_simple_display_pipe *pipe;
 
 	pipe = container_of(plane, struct drm_simple_display_pipe, plane);
-	if (!pipe->funcs || !pipe->funcs->prepare_fb)
-		return 0;
+	if (!pipe->funcs || !pipe->funcs->prepare_fb) {
+		if (WARN_ON_ONCE(!drm_core_check_feature(plane->dev, DRIVER_GEM)))
+			return 0;
+
+		WARN_ON_ONCE(pipe->funcs && pipe->funcs->cleanup_fb);
+
+		return drm_gem_simple_display_pipe_prepare_fb(pipe, state);
+	}
 
 	return pipe->funcs->prepare_fb(pipe, state);
 }

include/drm/drm_simple_kms_helper.h

···
 	 * the documentation for the &drm_plane_helper_funcs.prepare_fb hook for
 	 * more details.
 	 *
-	 * Drivers which always have their buffers pinned should use
-	 * drm_gem_simple_display_pipe_prepare_fb() for this hook.
+	 * For GEM drivers who neither have a @prepare_fb nor @cleanup_fb hook
+	 * set drm_gem_simple_display_pipe_prepare_fb() is called automatically
+	 * to implement this. Other drivers which need additional plane
+	 * processing can call drm_gem_simple_display_pipe_prepare_fb() from
+	 * their @prepare_fb hook.
 	 */
 	int (*prepare_fb)(struct drm_simple_display_pipe *pipe,
 			  struct drm_plane_state *plane_state);