vt(4): Add vtbuf_dirty*_locked() to lock vtbuf once, not twice

In several functions, vtbuf_putchar() in particular, the lock on vtbuf
is acquired twice:
    1. once by the said functions;
    2. once in vtbuf_dirty().

Now, vtbuf_dirty_locked() and vtbuf_dirty_cell_locked() allow to
acquire that lock only once.

This improves the input speed of vt(4). To measure the gain, a
50,000-lines file was displayed on the console using cat(1). The time
taken by cat(1) is reported below:
    o  On amd64, with vt_vga:
        - before: 1.0"
        - after:  0.5"
    o  On sparc64, with creator_vt:
        - before: 13.6"
        - after:  10.5"

This is an MFC of r269780.