drm/vkms: update the current status of todo list

Update:
- debugging issues on igt testcases
- plane composition features: add primary plane improvements
- suggestions of good tasks to start working on vkms

Drop:
- syzkaller bug report:
  what triggered the warning was replaced by shmem functions at
  https://patchwork.freedesktop.org/patch/394614/
- overlay plane: this feature was added by
  https://patchwork.freedesktop.org/patch/430941/



Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210626092655.ghmmt2yux5klrne7@smtp.gmail.com