drm/i915: Use DIV_ROUND_CLOSEST for PWM calculations

Supposedly we would want to get the PWM output as close as possible to
the target, so let's round to closest.

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Suggested-by: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1456932138-14004-7-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>