drm/i915/selftests: exercise mmap migration

Exercise each of the migration scenarios, verifying that the final
placement and buffer contents match our expectations.

v2(Thomas): Replace for_i915_gem_ww() block with simpler object_lock()

v3:
- For testing purposes allow forcing the io_size such that we can
  exercise the allocation + migration path on devices that don't have the
  small BAR limit.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220228123607.580432-4-matthew.auld@intel.com