drm/i915: Fix unsigned overflow when calculating total data rate, v2.

drm/i915: Fix unsigned overflow when calculating total data rate, v2.

On gen11, we can definitely smash the 32-bits barrier with just a
when we enable all planes in the next patch.

Changes since v1:
- Use div64_u64 (ickle).

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181022102000.30255-1-maarten.lankhorst@linux.intel.com