drm/i915: PLL defines for VLV

Add register definitions for the new VLV PLL bits.

v2: remove unused bits & regs (Ben)

Acked-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>