drm: Remove complete task from TODO documentation

This patch remove the follow complete task from TODO documentation:

  drm_mode_config.crtc_idr is misnamed, since it contains all KMS object.
  Should be renamed to drm_mode_config.object_idr.

Signed-off-by: Shayenne da Luz Moura <shayenneluzmoura@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181213225537.nq4dwidn6tma33iv@smtp.gmail.com