[FIX] stock: fix view of locked picking

When the multi-location is enabled, in the stock picking form, when
the picking is unlocked, we can't modify move lines in the wizard,
due to a bad readonly attribute which doesn't take in account
'is_locked' field.

closes odoo/odoo#44460

X-original-commit: cc2bff6d
Signed-off-by: Rémy Voet <ryv-odoo@users.noreply.github.com>