drm/plane: Move drm_plane_enable_fb_damage_clips into core

We're trying to have a fairly strict split between core functionality
that defines the uapi, including the docs, and the helper functions to
implement it.

Move drm_plane_enable_fb_damage_clips and associated kerneldoc into
drm_plane from drm_damage_helper.c to fix this.

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/20210723083457.696939-3-daniel.vetter@ffwll.ch