dim: use ${1:?$usage} for create-workdir argument checking

The benefit of ${1:?$usage} is two-fold. First, it allows enabling of
set -u without errors about unbound variable references. Second, it
provides a uniform error message about missing arguments.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>