drm/amdgpu:fix world switch hang

for SR-IOV, we must keep the pipeline-sync in the protection
of COND_EXEC, otherwise the command consumed by CPG is not
consistent when world switch triggerd, e.g.:

world switch hit and the IB frame is skipped so the fence
won't signal, thus CP will jump to the next DMAframe's pipeline-sync
command, and it will make CP hang foever.

after pipelin-sync moved into COND_EXEC the consistency can be
guaranteed

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>