drm/amdgpu: check flush fence context instead of same ring v2

Otherwise we can run into the following situation:

1. Process A grabs ID 1 for ring 0.
2. Process B grabs ID 1 for ring 0.
3. Process A grabs ID 1 for ring 1.
4. Process A tries to reuse ID1 for ring 0 but things he doesn't need to flush.

v2: check the context of the flush fence instead of messing with the owner field.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>