modesetting: Fix potential buffer overflow

If one misconfigures a ZaphodHeads value (more than 20 characters
without a delimiter), we get an overflow of our buffer.  Use
xstrtokenize() instead of writing/fixing our own tokenizer.

Signed-off-by: Daniel Martin <consume.noise@gmail.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
(cherry picked from commit 04a30512)