drm/i915: Use fixed_16_16 wrapper for division operation

Don't use fixed_16_16 structure members directly, instead use wrapper to
perform fixed_16_16 division operation.

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170517115831.13830-4-mahesh1.kumar@intel.com