Control whether to build SVM support into amdgpu with a Kconfig option.
This makes it easier to disable it in production kernels if this new
feature causes problems in production environments.

Use "depends on" instead of "select" for DEVICE_PRIVATE, as is
recommended for visible options.
Reviewed-by: Philip Yang <Philip.Yang@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>