drm/i915/vlv: add VLV specific clock_get function v3

Calculation is a little different than other platforms.

v2: update to use port_clock instead
    rebase on top of Ville's changes
v3: update to new port_clock semantics - don't divide by
    pixel_multiplier (Ville)

References: https://bugs.freedesktop.org/show_bug.cgi?id=67345
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>