drm/amdkfd: fix a potential ttm->sg memory leak

Memory is allocated for ttm->sg by kmalloc in kfd_mem_dmamap_userptr,
but isn't freed by kfree in kfd_mem_dmaunmap_userptr. Free it!

Fixes: 264fb4d3 ("drm/amdgpu: Add multi-GPU DMA mapping helpers")

Signed-off-by: Lang Yu <lang.yu@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>