drm/vkms: update todo

Drop issues already resolved in vkms:

- CRC API Improvements to [1] add igt test to check extreme alpha values
  and [2] alpha blending;
- [3] prime buffer sharing;
- [4] writeback support;

On the other hand, we also found or thought about other improvements since
the last update of this document:

- better support for IGT tests
- improvements to writeback support
- syzbot report

Finally, we reorder items by the assumed complexity.

[1] https://patchwork.freedesktop.org/series/55944/
[2] https://patchwork.freedesktop.org/series/80823/
[3] https://patchwork.freedesktop.org/series/63212/
[4] https://patchwork.freedesktop.org/series/81177/



v2:
- Link to syzbot dashboard

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Cc: Haneen Mohammed <hamohammed.sa@gmail.com>

Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201006223006.gu55pjtuaigkh6il@smtp.gmail.com