drm/amdgpu: cond_exec only for schedule with a job

issue:
under SR-IOV sometimes the iB test will fail on
gfx ring

fix:
with cond_exec inserted in RB the gfx engine would
skip part packets if RLCV issue PREEMPT on gfx engine
if gfx engine is prior to COND_EXEC packet, this is
okay for regular command from UMD, but for the ib test
since the whole dma format doesn't support PREEMPT
so must remove the COND_EXEC from it.

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