Forgot to update the docs when nuking that. Oops.

Fixes: fcee5906 ("drm: Nuke ->vblank_disable_allowed")
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1464093915-3935-1-git-send-email-daniel.vetter@ffwll.ch