auxdisplay: small cleanups

- Use BUILD_BUG_ON for CFAG12864B_SIZE instead of runtime-check

- Use get_zeroed_page()

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Miguel Ojeda Sandonis <maxextreme@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>