drm/i915/fbdev: Fix num_connector references in intel_fb_initial_config()

During boot time, MST devices usually send a ton of hotplug events
irregardless of whether or not any physical hotplugs actually occurred.
Hotplugs mean connectors being created/destroyed, and the number of DRM
connectors changing under us. This isn't a problem if we use
fb_helper->connector_count since we only set it once in the code,
however if we use num_connector from struct drm_mode_config we risk it's
value changing under us. On top of that, there's even a chance that
dev->mode_config.num_connector != fb_helper->connector_count. If the
number of connectors happens to increase under us, we'll end up using
the wrong array size for memcpy and start writing beyond the actual
length of the array, occasionally resulting in kernel panics.

Note: This is just polish for 4.7, Dave Airlie's drm_connector
refcounting fixed these bugs for real. But it's good enough duct-tape
for stable kernel backporting, since backporting the refcounting
changes is way too invasive.

Cc: stable@vger.kernel.org
Signed-off-by: Lyude <cpaul@redhat.com>
[danvet: Clarify why we need this.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1463065021-18280-2-git-send-email-cpaul@redhat.com

drivers/gpu/drm/i915/intel_fbdev.c

@@ -368,12 +368,12 @@ static bool intel_fb_initial_config(struct drm_fb_helper *fb_helper,
 	uint64_t conn_configured = 0, mask;
 	int pass = 0;
 
-	save_enabled = kcalloc(dev->mode_config.num_connector, sizeof(bool),
+	save_enabled = kcalloc(fb_helper->connector_count, sizeof(bool),
 			       GFP_KERNEL);
 	if (!save_enabled)
 		return false;
 
-	memcpy(save_enabled, enabled, dev->mode_config.num_connector);
+	memcpy(save_enabled, enabled, fb_helper->connector_count);
 	mask = (1 << fb_helper->connector_count) - 1;
 retry:
 	for (i = 0; i < fb_helper->connector_count; i++) {
@@ -520,7 +520,7 @@ static bool intel_fb_initial_config(struct drm_fb_helper *fb_helper,
 	if (fallback) {
 bail:
 		DRM_DEBUG_KMS("Not using firmware configuration\n");
-		memcpy(enabled, save_enabled, dev->mode_config.num_connector);
+		memcpy(enabled, save_enabled, fb_helper->connector_count);
 		kfree(save_enabled);
 		return false;
 	}