dim: abstract and fix maintainer scope check for dinq

Separate maintainer scope check from checkpatch, and only do the check
on applying patches. Also, fix the failures on grep not matching.

Fixes: 56e53a49 ("dim: declare and assign separately")
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>