drm/i915: Always try to inherit the initial fb.

The initial state is read out correctly and the state is atomic,
so it's safe to preserve the fb without any hacks if it's suitable.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>