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 <sam@ravnborg.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210623162456.3373469-1-daniel.vetter@ffwll.ch

include/drm/drm_simple_kms_helper.h

@@ -116,8 +116,11 @@ struct drm_simple_display_pipe_funcs {
 	 * 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);