drm/i915: alloc intel_fb in the intel_fbdev struct

Allocate this struct instead, so we can re-use another allocated
elsewhere if needed.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: WARN_ON if there's no backing storage attached to an fb,
that's a bug.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>