drm/amdgpu: only keep most recent fence for each context

drm/amdgpu: only keep most recent fence for each context

Keep only the latest fences to reduce the number of values
given back to userspace

v2: - Export this code from dma-fence-unwrap.c(by Christian).
v3: - To split this in a dma_buf patch and amd userq patch(by Sunil).
    - No need to add a new function just re-use existing(by Christian).
v4: Export dma_fence_dedub_array function and used it(by Christian).

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Reviewed-by: Sunil Khatri <sunil.khatri@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Arvind Yadav <Arvind.Yadav@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>