drm/i915/gem: Introduce a selftest for the gem object migrate functionality

A selftest for the gem object migrate functionality. Slightly adapted
from the original by Matthew to the new interface and new fill blit
code.

v4:
- Initialize buffers and check contents after migration
  (Suggested by Matthew Auld)
- Perform async migration (if implemented) in the igt_lmem_pages_migrate
  test
- Test also migration to the current region.

Co-developed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com> #v3
Link: https://patchwork.freedesktop.org/patch/msgid/20210629151203.209465-3-thomas.hellstrom@linux.intel.com