drm/atomic: trim driver interface/docs

Remove the kerneldoc and EXPORT_SYMBOL which aren't used and really
shouldn't ever be used by drivers directly.

Unfortunately this means we need to move the set_writeback_fb function
around to avoid a forward decl.

Acked-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180905135711.28370-5-daniel.vetter@ffwll.ch