drm/i915: get_plane_config support for ILK+ v3

This should allow BIOS fb inheritance to work on ILK+ machines too.

v2: handle tiled BIOS fbs (Kristian)
    split out common bits (Jesse)
v3: alloc fb obj out in _init

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