drm/doc: overview before functions for drm_writeback.c

Otherwise it's really hard to link to that, which I realized when I
wanted to link to the property definitions for a question on irc.

Fix it.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Fixes: e2d7fc20 ("drm/writeback: wire drm_writeback.h to kernel-doc")
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: 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>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220111202714.1128406-1-daniel.vetter@ffwll.ch