auxdisplay: hd44780: Introduce hd44780_common_free()

Introduce hd44780_common_free() for symmetrical operation
to hd44780_common_alloc(). It will allow to modify both
in the future without touching the users.

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