drm/i915/ttm: fix sg_table construction

If we encounter some monster sized local-memory page that exceeds the
maximum sg length (UINT32_MAX), ensure that don't end up with some
misaligned address in the entry that follows, leading to fireworks
later. Also ensure we have some coverage of this in the selftests.

v2(Chris):
  - Use round_down consistently to avoid udiv errors
v3(Nirmoy):
  - Also update the max_segment in the selftest

Fixes: f701b16d

 ("drm/i915/ttm: add i915_sg_from_buddy_resource")
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/6379
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Nirmoy Das <nirmoy.das@linux.intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220711085859.24198-1-matthew.auld@intel.com