drm/amdgpu: DROP redundant drm_prime_sg_to_dma_addr_array

For DMA-MAP userptr on other GPUs, the dma address array
will be populated in amdgpu_ttm_backend_bind.

Remove the redundant call from the driver.

v2:
  update the comment

Signed-off-by: Shane Xiao <shane.xiao@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>