drm/amd/display: Handle case when stream not found in set_dpms

When validate with context fail to add stream to the context, we have
a case where set_dpms won't be able to find the stream that need to
be disabled.

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>