drm/amdgpu: add new low overhead command submission API. (v2)

This just sends chunks to the kernel API for a single command
stream.

This should provide a more future proof and extensible API
for command submission.

v2: use amdgpu_bo_list_handle, add two helper functions to
access bo and context internals.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>