drm/i915: Remove 10% cdclk guardband on BXT

We don't need any pixel clock vs. cdclk guardband since HSW. BXT still
tries to add one though. Get rid of it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1462995892-32416-8-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>