drm/i915: VLV GPU frequency to opcode functions

When requesting frequency changes or querying status from the Punit, we
need to use an opcode that corresponds to the frequency, taking into
account the memory frequency.

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