drm/amdgpu: cleanup in kernel job submission

Add a job_alloc_with_ib helper and proper job submission.

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