drm/i915: Use 64-bit division macro

Since the PWM framework is switching struct pwm_state.duty_cycle's
datatype to u64, prepare for this transition by using DIV_ROUND_UP_ULL
to handle a 64-bit dividend.

Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>