drm/i915: Round up when calculating display bandwidth requirements

We should round up when doing bandwidth calculations to make sure
our estimates don't fall short of the actual number.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220303191207.27931-7-ville.syrjala@linux.intel.com


Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>