drm/amdkfd: Get prange->offset after svm_range_vram_node_new

During miration to vram prange->offset is valid after vram buffer is located,
either use old one or allocate a new one. Move svm_range_vram_node_new before
migrate for each vma to get valid prange->offset.

v2: squash in warning fix

Fixes: b4ee9606 ("drm/amdkfd: Fix BO offset for multi-VMA page migration")
Signed-off-by: Xiaogang Chen <Xiaogang.Chen@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>