drm/i915: Fix oops caused by fbdev initialization failure

intelfb_create() is called once on driver initialization. If it fails,
ifbdev->helper.fbdev, ifbdev->fb or ifbdev->fb->obj may be NULL.

Further up in the call stack, intel_fbdev_initial_config() calls
intel_fbdev_fini() to tear down the ifbdev on failure. This calls
intel_fbdev_destroy() which dereferences ifbdev->fb. Fix the ensuing
oops.

Also check in these functions if ifbdev is not NULL to avoid oops:

i915_gem_framebuffer_info() is called on access to debugfs file
"i915_gem_framebuffer" and dereferences ifbdev, ifbdev->helper.fb
and ifbdev->helper.fb->obj.

intel_connector_add_to_fbdev() / intel_connector_remove_from_fbdev()
are called when registering / unregistering an mst connector and
dereference ifbdev.

v3: Drop additional null pointer checks in intel_fbdev_set_suspend(),
    intel_fbdev_output_poll_changed() and intel_fbdev_restore_mode()
    since they already check if ifbdev is not NULL, which is sufficient
    now that intel_fbdev_fini() is called on initialization failure.
    (Requested by Daniel Vetter <daniel.vetter@ffwll.ch>)
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: http://patchwork.freedesktop.org/patch/msgid/d05f0edf121264a9d0adb8ca713fd8cc4ae068bf.1447938059.git.lukas@wunner.deSigned-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -1878,6 +1878,7 @@ static int i915_gem_framebuffer_info(struct seq_file *m, void *data)
 	struct drm_i915_private *dev_priv = dev->dev_private;
 
 	ifbdev = dev_priv->fbdev;
+	if (ifbdev) {
 		fb = to_intel_framebuffer(ifbdev->helper.fb);
 
 		seq_printf(m, "fbcon size: %d x %d, depth %d, %d bpp, modifier 0x%llx, refcount %d, obj ",
@@ -1889,6 +1890,7 @@ static int i915_gem_framebuffer_info(struct seq_file *m, void *data)
 			   atomic_read(&fb->base.refcount.refcount));
 		describe_obj(m, fb->obj);
 		seq_putc(m, '\n');
+	}
 #endif
 
 	mutex_lock(&dev->mode_config.fb_lock);

drivers/gpu/drm/i915/intel_dp_mst.c

@@ -408,7 +408,10 @@ static void intel_connector_add_to_fbdev(struct intel_connector *connector)
 {
 #ifdef CONFIG_DRM_FBDEV_EMULATION
 	struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
-	drm_fb_helper_add_one_connector(&dev_priv->fbdev->helper, &connector->base);
+
+	if (dev_priv->fbdev)
+		drm_fb_helper_add_one_connector(&dev_priv->fbdev->helper,
+						&connector->base);
 #endif
 }
 
@@ -416,7 +419,10 @@ static void intel_connector_remove_from_fbdev(struct intel_connector *connector)
 {
 #ifdef CONFIG_DRM_FBDEV_EMULATION
 	struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
-	drm_fb_helper_remove_one_connector(&dev_priv->fbdev->helper, &connector->base);
+
+	if (dev_priv->fbdev)
+		drm_fb_helper_remove_one_connector(&dev_priv->fbdev->helper,
+						   &connector->base);
 #endif
 }
 

drivers/gpu/drm/i915/intel_fbdev.c

@@ -529,8 +529,10 @@ static void intel_fbdev_destroy(struct drm_device *dev,
 
 	drm_fb_helper_fini(&ifbdev->helper);
 
+	if (ifbdev->fb) {
 		drm_framebuffer_unregister_private(&ifbdev->fb->base);
 		drm_framebuffer_remove(&ifbdev->fb->base);
+	}
 }
 
 /*