drm/doc: Fix vga_switcheroo xref

Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: b754b35b ("vgaarbiter: rst-ifiy and polish kerneldoc")
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160824101916.30125-1-daniel.vetter@ffwll.ch