[gdb/tui] Refactor prefresh call in tui_source_window_base::refresh_window

[gdb/tui] Refactor prefresh call in tui_source_window_base::refresh_window

Currently the call to prefresh in tui_source_window_base::refresh_window looks
like:
...
  prefresh (m_pad.get (), 0, pad_x, y + 1, x + left_margin,
	    y + m_content.size (), x + left_margin + view_width - 1);
...

This is hard to parse.  It's not obvious what the arguments mean, and there's
repetition in the argument calculation.

Fix this by rewriting the call as follows:
- use sminrow, smincol, smaxrow and smaxcol variables for the last
  4 arguments, and
- calculate the smaxrow and smaxcol variables based on the sminrow and
  smincol variables.

Tested on x86_64-linux.

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