drm/i915: read current freq from Punit on VLV

Instead of returning the cached value, which is just what the kernel
requested.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>