drm/i915: Wait for cdclk change to occure when going for 400MHz

VLV Punit doesn't support the 400MHz cdclk option, so we bypass the
Punit and poke at CCK directly. However we forgot to wait for the
frequeency change to complete. Poll the CCK clock status to make sure
the clock has changed before we fire up any pipes.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>