video: fbdev: atari: Fix TT High video mode

The horizontal resolution (640) for the TT High video mode (1280x960) is
definitely bogus.  While fixing that, correct the timings to match the
TTM195 service manual.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Helge Deller <deller@gmx.de>