drm/radeon: only increment sync_seq when a fence is really emitted

In the rare situation where the kmalloc fails we're probably screwed anyway,
but let's try to be more robust about it.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Nicolai Hähnle <Nicolai.Haehnle@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>