drm/qxl: Don't register debugfs for control minors

They're gone since 8a357d10 ("drm: Nerf DRM_CONTROL nodes").
Spotted while doing a full audit when revieng a similar patch from
Nicolai for radeon.

v2: Drink coffee first aka don't forget the unregister side.

Cc: Dave Airlie <airlied@redhat.com>
Fixes: 8a357d10 ("drm: Nerf DRM_CONTROL nodes")
Cc: Nicolai Stange <nicstange@gmail.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161205072926.12546-1-daniel.vetter@ffwll.ch