drm/amdgpu: Add kgd/kfd interface to support scratch memory v2

v2:
* Shortened headline
* Removed write_config_static_mem, it gets initialized by gfx_v?_0_gpu_init
* Renamed alloc_memory_of_scratch to set_scratch_backing_va
* Made set_scratch_backing_va a void function
* Documented set_scratch_backing in kgd_kfd_interface.h

Signed-off-by: Moses Reuben <moses.reuben@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>