[gdb/tui] Use more box_width in tui-regs.c

[gdb/tui] Use more box_width in tui-regs.c

While experimenting with can_box () == false by default, I noticed two places
in tui-regs.c where we can replace a hardcoded 1 with box_width ().

It also turned out to be necessary to set scrollok to false, otherwise writing
the last char of the last line with register info will cause a scroll.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>