drm/amdgpu: add ctx_id to the WAIT_CS IOCTL (v4)

It is required to support fence per context.

v2: add amdgpu_ctx_get/put
v3: improve get/put
v4: squash hlock fix

Signed-off-by: Jammy Zhou <Jammy.Zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>