drm/i915/gvt: Replace roundup with DIV_ROUND_UP

It's a bit redundant for the maths here using roundup.

Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Tested-by: Terrence Xu <terrence.xu@intel.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Link: https://lore.kernel.org/r/20220723020256.30081-3-nicolinc@nvidia.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>