drm/amdgpu: fix use-after-unlock in eviction fence destroy

drm/amdgpu: fix use-after-unlock in eviction fence destroy

The eviction fence destroy path incorrectly calls dma_fence_put() on
evf_mgr->ev_fence after releasing the ev_fence_lock. This introduces a
potential use-after-unlock or race because another thread concurrently
modifies evf_mgr->ev_fence.

Fix this by grabbing a local reference to evf_mgr->ev_fence under the
lock and using that for dma_fence_put() after waiting.

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