drm/i915: implement crtc helper semantics relied upon by the fb helper

Yikes!

But yeah, we have to do this until someone volunteers to clean up the
fb helper and rid it of its incetious relationship with the crtc
helper code.
Reviewed-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -6934,6 +6934,12 @@ static int intel_crtc_set_config(struct drm_mode_set *set)
 	if (!set->mode)
 		set->fb = NULL;
 
+	/* The fb helper likes to play gross jokes with ->mode_set_config.
+	 * Unfortunately the crtc helper doesn't do much at all for this case,
+	 * so we have to cope with this madness until the fb helper is fixed up. */
+	if (set->fb && set->num_connectors == 0)
+		return 0;
+
 	if (set->fb) {
 		DRM_DEBUG_KMS("[CRTC:%d] [FB:%d] #connectors=%d (x y) (%i %i)\n",
 				set->crtc->base.id, set->fb->base.id,