drm/amdgpu/userq: actually validate userq BOs

This was supposed to loop over the individual evicted list, as the
kernel and always_valid evicted lists were already handled in
amdgpu_vm_validate.

Without this, BOs that were created and evicted before a userq was made
would never end up in an eviction fence rearm.

Also add the missing ref to the BO before drm_exec locks and refs it.

Fixes: 4cdbba5a16aa ("drm/amdgpu: restructure VM state machine v4")
Signed-off-by: Benjamin Cheng <benjamin.cheng@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 18decfc9e8cc0fa70b6de9944cc27a37729edfbd)
Cc: stable@vger.kernel.org