drm/atomic-helper: Reject legacy flips on a disabled pipe

We want this for consistency with existing page_flip semantics.

Since this spurred quite a discussion on IRC also document why we
reject event generation when the pipe is off: It's not that it's hard
to implement, but userspace has a track recording which proves that it's
way too easy to accidentally abuse and cause havoc. We want to make
sure userspace doesn't get away with that.

v2: Somehow thought we do reject events already, but that code only
existed in my imagination ... Also suggestions from Thierry.

Cc: Daniel Stone <daniels@collabora.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449564561-3896-4-git-send-email-daniel.vetter@ffwll.ch

include/drm/drm_crtc.h

@@ -551,7 +551,8 @@ struct drm_crtc_funcs {
 	 * ->page_flip() operation is already pending the callback should return
 	 * -EBUSY. Pageflips on a disabled CRTC (either by setting a NULL mode
 	 * or just runtime disabled through DPMS respectively the new atomic
-	 * "ACTIVE" state) should result in an -EINVAL error code.
+	 * "ACTIVE" state) should result in an -EINVAL error code. Note that
+	 * drm_atomic_helper_page_flip() checks this already for atomic drivers.
 	 */
 	int (*page_flip)(struct drm_crtc *crtc,
 			 struct drm_framebuffer *fb,