drm/amdkfd: Fix an eviction fence leak

Only creating a new reference for each process instead of each VM.

Fixes: 9a1c1339 ("drm/amdkfd: Run restore_workers on freezable WQs")
Suggested-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Lang Yu <lang.yu@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 5fa43628)
Cc: stable@vger.kernel.org