drm/i915/bdw: display stuff

Just enough to make the code not barf...

Init BDW display to look like HSW. For the simulator this should be
fine, but this will probably require more work.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Add a FIXME comment about RCS flips being untested on bdw.
Also add a note that hblank events are reserved on bdw+ in DERRMR.]
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>