drm: Add a STEREO_3D capability to the SET_CLIENT_CAP ioctl

This capability allows user space to control the delivery of modes with
the 3D flags set. This is to not play games with current user space
users not knowing anything about stereo 3D flags and that could try
to set a mode with one or several of those bits set.

So, the plan is to remove the stereo modes from the list of modes we
give to DRM clients by default, and let them through if we are being
told otherwise.

stereo_allowed is bound to the drm_file structure to make it a
per-client setting, not a global one.

v2: Replace clearing 3D flags by discarding the stereo modes now that
    they are regular modes.
v3: SET_CAP -> SET_CLIENT_CAP rename (Chris Wilson)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>