drm/writeback: wire drm_writeback.h to kernel-doc

drm_writeback.h included a lot of nice kernel-doc comments.
Wire it up so the header file is included in the kernel-doc
generated documentation.

Added a few simple comments to the two structs so they
get picked up by kernel-doc.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Cc: Brian Starkey <brian.starkey@arm.com>
Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200406194746.26433-4-sam@ravnborg.org