drm/amdkfd: use resource cursor in svm_migrate_copy_to_vram v2

Access to the mm_node is now forbidden. So instead of hand wiring that
use the cursor functionality.

v2: fix handling as pointed out by Philip.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by and Tested-by: Philip Yang <philip.yang@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210602100914.46246-5-christian.koenig@amd.com