drm/amdgpu: stop using the ring index in the SA

The ring index will always collide as hash into the fence list, so use
the context number instead. That can still cause collisions, but they
are less likely than using ring indices.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>