dim: s/which/command -v/

gitlab builder fails

Also I think it's high time we switch over to gitlab pull requests to
stop stuff like this, because locally make check was all fine.

Cc: Jani Nikula <jani.nikula@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>