drm/i915/bxt: Set max cdclk frequency properly

intel_update_max_cdclk() doesn't have a switch case for Broxton, so
dev_priv->max_cdclk_freq gets set to whatever clock frequency we're
currently running at (e.g., 144 MHz) rather than the true maximum.  This
causes our max dotclock to also be set too low and in turn leads mode
verification to reject perfectly valid modes while loading EDID firmware
blobs.

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459892239-14041-1-git-send-email-matthew.d.roper@intel.com