drm/i915: Fix build warning on 32-bit

The gtt.stolen_size field is of type size_t, and so should be printed
using %zu to avoid build warnings on either 32-bit and 64-bit builds.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>