drm/i915: Pass computed vco to bxt_set_cdclk()

Rather than compute the vco inside bxt_set_cdclk() let's precompute it
outside and pass it in. A small step towards a fully precomputed cdclk
state.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170120182205.8141-7-ville.syrjala@linux.intel.com