drm/amdgpu: Add mapping info option for GEM_OP ioctl

Add new GEM_OP_IOCTL option GET_MAPPING_INFO, which
returns a list of mappings associated with a given bo, along with
their positions and offsets.

Userspace for this and the previous change can be found at:
https://github.com/checkpoint-restore/criu/pull/2613



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