dim: sanity-check config settings

Shashank wanted to reuse his drm-tip repo for DIM_REPO, which doesn't
work great. Catch this.

Also group the various check functions all together for a bit of OCD.

Cc: Shashank Sharma <shashank.sharma@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>