drm/i915: Correct the Ref clock value for BXT

The reference clock for BXT is 19.2 MHz not 19.5 MHz, updating the
correct value here.

Signed-off-by: Deepak M <m.deepak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449238659-12473-2-git-send-email-m.deepak@intel.com