dim: s/DIM_DRM_INTEL/DIM_REPO/

The maintainer repo hasn't been Intel specific for a while now. Rename
the variable. DIM_DRM_INTEL is used as a default for DIM_REPO for
backward compatibility.

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