drm/nouveau: add kconfig option to turn off nouveau legacy contexts. (v3)

There was a nouveau DDX that relied on legacy context ioctls to work,
but we fixed it years ago, give distros that have a modern DDX the
option to break the uAPI and close the mess of holes that legacy
context support is.

Full context of the story:

commit 0e975980
Author: Peter Antoine <peter.antoine@intel.com>
Date:   Tue Jun 23 08:18:49 2015 +0100

    drm: Turn off Legacy Context Functions

    The context functions are not used by the i915 driver and should not
    be used by modeset drivers. These driver functions contain several bugs
    and security holes. This change makes these functions optional can be
    turned on by a setting, they are turned off by default for modeset
    driver with the exception of the nouvea driver that may require them with
    an old version of libdrm.

    The previous attempt was

    commit 7c510133
    Author: Daniel Vetter <daniel.vetter@ffwll.ch>
    Date:   Thu Aug 8 15:41:21 2013 +0200

        drm: mark context support as a legacy subsystem

    but this had to be reverted

    commit c21eb21c
    Author: Dave Airlie <airlied@redhat.com>
    Date:   Fri Sep 20 08:32:59 2013 +1000

        Revert "drm: mark context support as a legacy subsystem"

    v2: remove returns from void function, and formatting (Daniel Vetter)

    v3:
    - s/Nova/nouveau/ in the commit message, and add references to the
      previous attempts
    - drop the part touching the drm hw lock, that should be a separate
      patch.

    Signed-off-by: Peter Antoine <peter.antoine@intel.com> (v2)
    Cc: Peter Antoine <peter.antoine@intel.com> (v2)
Reviewed-by: Peter Antoine <peter.antoine@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

v2: move DRM_VM dependency into legacy config.
v3: fix missing dep (kbuild robot)

Cc: stable@vger.kernel.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>