drm/i915: Rewrite broxton_get_display_clock_speed() in terms of the DE PLL vco/refclk

Now that we've read out the DE PLL vco and refclk, we can just use them
in the cdclk calculation. While at it switch over to
DIV_ROUND_CLOSEST().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463172100-24715-19-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>