drm/i915/selftests: Fixup naked 64b divide

drivers/gpu/drm/i915/intel_memory_region.o: in function `igt_mock_contiguous':
drivers/gpu/drm/i915/selftests/intel_memory_region.c:166: undefined reference to `__umoddi3'

v2: promote target to u64 for consistency across all builds

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: 2f0b97ca

 ("drm/i915/region: support contiguous allocations")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191013114509.3405-1-chris@chris-wilson.co.uk