drm/i915: Use DIV_ROUND_CLOSEST()

vlv_find_best_dpll() has an open coded DIV_ROUND_CLOSEST(). Replace it
with the real thing.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>