drm: docs: Update task from drm TODO list

drm: docs: Update task from drm TODO list

Update TODO item from drm documentation to contain more applicable
information regarding the removal of deprecated MIPI DSI functions and
no longer reference functions which have already been removed from the
kernel.

Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Brigham Campbell <me@brighamcampbell.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20250722015313.561966-5-me@brighamcampbell.com