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>