drm/<drivers>: don't set driver->dev_priv_size to 0

Especially not on modesetting drivers - this is used to size
the driver private structure for legacy drm buffers.

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>