drm/amdgpu: validate VM PTs only on eviction

We don't need to validate them again if the eviction counter didn't changed.

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