auxdisplay: linedisp: Free allocated resources in ->release()

While there is no issue currently with the resources allocation,
the code may still be made more robust by deallocating message
in the ->release() callback.

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>