drm/i915/ttm: Update i915_gem_obj_copy_ttm() to be asynchronous

Update the copy function i915_gem_obj_copy_ttm() to be asynchronous for
future users and update the only current user to sync the objects
as needed after this function.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211122214554.371864-7-thomas.hellstrom@linux.intel.com