drm/i915: don't preserve inherited configs with nothing on v2

It can be corrected later and may be what was actually desired, but
generally isn't, so if we find nothing is enabled, let the core DRM fb
helper figure something out.

v2: free the array too (Jesse)

Note that this also undoes any changes in case we bail out due to hw
cloning.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>