drm/amdgpu: Fix KFD initialization for multi-GPU systems

kfd2kgd is device-specific, so it should not be a global variable.
Merge amdgpu_amdkfd_load_interface and amdgpu_amdkfd_device_probe
so that it's only needed as a local variable in one function.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>