drm/i915/bdw: enable eDRAM.

The same register exists for querying and programming eDRAM AKA eLLC. So
we can simply use it. For now, use all the same defaults as we had
for Haswell, since like Haswell, I have no further details.

I do not actually have a part with eDRAM, so I cannot test this.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: Brad Volkin <bradley.d.volkin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>