drm/i915: Fix 66 MHz LVDS SSC freq for gen2

Store the SSC refclock frequency in kHz to get more accuracy. Currently
we're pretending that 66 MHz is ~66000 kHz, when in fact it is actually
~66667 kHz. By storing the less rounded kHz value we get a much better
accuracy for out pixel clock calculations.

Cc: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>