drm/amdgpu: fix a wrong ib reference

It should be p->job->ibs[j] instead of p->job->ibs[i] here.

Fixes: cdc7893f ("drm/amdgpu: use job and ib structures directly in CS parsers")
Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>