Use TUI_DISASM_WIN instead of tui_win_list array

This is a minor cleanup to change tui_get_low_disassembly_address to
use TUI_DISASM_WIN, rather than the tui_win_list array.  This is more
in line with what the rest of the TUI code does.

gdb/ChangeLog
2020-02-22  Tom Tromey  <tom@tromey.com>

	* tui/tui-disasm.c (tui_get_low_disassembly_address): Use
	TUI_DISASM_WIN, not tui_win_list.

Change-Id: I999335ee3f63a4b570e84f320236b78f2bd5b780