drm/i915: magic VLV PLL registers in the dpio sideband

Stolen from a patch with the below impressive sob-section.

Signed-off-by: Pallavi G <pallavi.g@intel.com>
Signed-off-by: Vijay Purushothaman <vijay.a.purushothaman@intel.com>
Signed-off-by: Gajanan Bhat <gajanan.bhat@intel.com>
Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: Drop everything but the header #defines.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>