drm/i915/bdw: Initialize BDW forcewake vfuncs

Somehow this got missed or dropped during development. The simulator
does not use forcewake, so it's entirely possible it never worked
correctly. After the mmio rework, this will end up in an OOPs, and the
system will not boot.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Use IS_GEN8 instead of IS_BROADWELL.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>