drm/simple-helper: drm_gem_simple_display_pipe_prepare_fb as default

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