drm/atomic-helpers: Clear up cleanup_done a bit

It's not obvious at first sight that this is a fastpath, make that
clearer with a goto. Fallout from a discussion with Liviu on irc.

v2: Drop bogus hunks that crept in.

v3: Make it compile.

Cc: Liviu.Dudau@arm.com
Acked-by:  <Liviu.Dudau@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465986266-2831-1-git-send-email-daniel.vetter@ffwll.ch