fbdev: atafb - Fix line length handling

  - Make sure par->next_line is always set (this was done for Falcon only),
    as all the text console drawing operations need a valid par->next_line,
  - Make sure fix->line_length is always set, as some userspace applications
    need it because they don't have fallback code for the case where it's zero.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>