drm/plane: remove drm_helper_get_plane_damage_clips

It's not used. Drivers should instead use the helpers anyway.

Currently both vbox and i915 hand-roll this and it's not the greatest.
vbox looks buggy, and i915 does a bit much that helpers would take
care of I think.

Also improve the kerneldocs while we're at it.

Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Hans de Goede <hdegoede@redhat.com>
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/20210723083457.696939-1-daniel.vetter@ffwll.ch