drm/sis: track obj->drm_fd relations in the driver

By attach a driver private struct to each open drm fd.

Because we steal the owner_list from drm_sman until things settle,
use list_move instead of list_add.

This requires to export a drm_sman function temporarily before
drm_sman will die for real completely.

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