dim: don't fall over relative names

If DIM_DRM_INTEL has .. in it (i.e. relative path) not everything
worked as expected, this fixes it. Allows you to use an existing git
checkout, while stashing all the dim stuff into some subdir somewhere
out of sight.

v2: Fixup paths. Oops.

v3: Fixup misplace hunk from rebasing.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>