drm: Update todo.rst

- drmP.h is now fully split up.
- vkms is happening (and will gain its own todo and docs under a new
  vkms.rst file real soon)
- legacy cruft is completely hidden now, drm_vblank.c is split out
  from drm_irq.c now. I've decided to drop the task to split out
  drm_legacy.ko, partially because Dave already rejected a patch to
  hide the old dri1 drivers better. Current state feels good enough to
  me.
- best_encoder atomic cleanup is done (it's now the default, not even
  exported anymore)
- bunch of smaller things

v2:
- Explain why the drm_legacy.ko task is dropped (Emil).
- typos (Sam).

v3: Fix typo (Ilia)

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: David Airlie <airlied@linux.ie>
Link: https://patchwork.freedesktop.org/patch/msgid/20180905181509.19530-1-daniel.vetter@ffwll.ch