drm/amdkfd: avoid unmap dma address when svm_ranges are split

DMA address reference within svm_ranges should be unmapped only after
the memory has been released from the system. In case of range
splitting, the DMA address information should be copied to the
corresponding range after this has split. But leaving dma mapping
intact.

Signed-off-by: Alex Sierra <alex.sierra@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>