drm/i915: add plane_config fetching infrastructure v2

Early at init time, we can try to read out the plane config structure
and try to preserve it if possible.

v2: alloc fb obj at init time after fetching plane config

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