drm/amdgpu: Relocate kgd2kfd function declaration

Since amdkfd is merged into amdgpu module and amdgpu can access amdkfd
directly, move declaration of kgd2kfd functions from kfd_priv.h to
amdgpu_amdkfd.h

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>