drm: Simplify and defend later checks when disabling a crtc

By setting the FB of a CRTC to NULL, we are turning off the CRTC (and so
disable the unused encoders and connectors). As such we can simplify the
later tests by making sure the set->mode is NULL. Setting the
num_connectors to zero means that we do not need to loop over the unused
connectors.

All current usage appears correct, this only builds additional defense
into the routine.

References: https://bugzilla.kernel.org/show_bug.cgi?id=27722
Tested-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/drm_crtc_helper.c

@@ -497,14 +497,17 @@ int drm_crtc_helper_set_config(struct drm_mode_set *set)
 
 	crtc_funcs = set->crtc->helper_private;
 
+	if (!set->mode)
+		set->fb = NULL;
+
 	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,
 				(int)set->num_connectors, set->x, set->y);
 	} else {
-		DRM_DEBUG_KMS("[CRTC:%d] [NOFB] #connectors=%d (x y) (%i %i)\n",
-				set->crtc->base.id, (int)set->num_connectors,
-				set->x, set->y);
+		DRM_DEBUG_KMS("[CRTC:%d] [NOFB]\n", set->crtc->base.id);
+		set->mode = NULL;
+		set->num_connectors = 0;
 	}
 
 	dev = set->crtc->dev;