drm/amdgpu: Simplify kgd2kfd interface

After amdkfd is merged into amdgpu module, amdgpu can call amdkfd
functions directly.

Signed-off-by: Amber Lin <Amber.Lin@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>