drm/via: use drm_mm instead of drm_sman

To make the transition in a piece-wise and bisectable way possible,
I've hijacked the ->owner_list from drm_sman. While transitioning, the
list_add was done by the driver, while the list_del was still done by
the dying sman code.

Now that we are in full control of ->owner_list, do the list_del
ourselves.

v2: Better explain the list_del trickery as suggested by Chris Wilson.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>