drm/todo: Fallout from v3d review

Bunch of ideas from Eric and me on what we could do to make gem gpu
rendering drivers a notch simpler to type.

v2: Fix typo (Eric).

Cc: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180425111742.5872-1-daniel.vetter@ffwll.ch