vt: Consistently take vtbuf lock for mouse cursor position

[quentin.thebault_defenso.fr]

Summary

vd_mx/vd_my and vd_mx_drawn/vd_my_drawn are read by vt_flush() under the
vtbuf lock, but the writers did not use that lock consistently, resulting
in races reported by KCSAN (see PR).

Take the vtbuf lock for every write and move the vd_mx_drawn/vd_my_drawn
read in vt_mark_mouse_position_as_dirty() inside the lock, matching
vt_flush().

PR: 298762
Sponsored by: Defenso

Signed-off-by: Quentin Thébault <[e-mail]>