drm/amdgpu: keep all fences in an RCU protected array v2

Just keep all HW fences in a RCU protected array as a
first step to replace the wait queue.

v2: update commit message, move fixes into separate patch.

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