drm/amdkfd: Separate dma unmap and free of dma address array operations

We do not need free dma address array of svm_range each time we do dma unmap
for pages in svm_range as we can reuse the same array. Only free it when free
svm_range. Separate these two operations and use them accordingly.

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>