radeon: fix regression with eviction since evict caching changes

Since 0d0b3e74
drm/radeon: use cached memory when evicting for vram on non agp

evicting from TTM would try and evict to TTM instead of system,
not so good.

This should fix:
https://bugs.freedesktop.org/show_bug.cgi?id=58272



Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>