drm/i915: don't save all the encoder/crtc state in set_config

We actually only touch the connector -> encoder and encoder -> crtc
linking. So it's enough to just save/restore that.

While at it, also switch to kcalloc to allocate these arrays (omission
in the commit message spotted by Jesse Barnes).

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