auxdisplay: linedisp: Allocate buffer for the string

Always allocate a buffer for the currently displayed characters.
It makes the line display API simpler.

Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>