drm/amdgpu: Replace kgd_mem with amdgpu_bo for kernel pinned gtt mem

The extra fields in struct kgd_mem aren't actually needed. This struct
will be used for GPUVM allocations later.

Signed-off-by: Yong Zhao <yong.zhao@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>