drm/amdkfd: Disable idle optimization for chained runlist

This works around difficult-to-reproduce soft hangs on oversubscribed
runlists.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Oak Zeng <Oak.Zeng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>