drm/amdgpu: don't require a job for cond_exec and shadow

We need to reset the shadow state every time we submit an
IB and there needs to be a COND_EXEC packet after the
SET_Q_PREEMPTION_MODE packet for it to work properly, so
we should emit both of these packets regardless of whether
there is a job present or not.

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