drm/amdgpu: add pre_soft_reset ip func

It will be used before soft_reset to do some preparing work for reset.

Signed-off-by: Chunming Zhou <David1.Zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>