drm/i915: Clear the stolen fb before enabling

As the stolen memory region will contain the contents of whatever was
last there, it invariably contains garbage. To be consistent with the
shmemfs backed fb and the expectations of the fb layer, we need to clear
the fb prior to installing it as an fbcon.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=58111
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Fixup sparse __iomem confusion reported by Wu Fengguang.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_fb.c

@@ -155,6 +155,13 @@ static int intelfb_create(struct intel_fbdev *ifbdev,
 	drm_fb_helper_fill_fix(info, fb->pitches[0], fb->depth);
 	drm_fb_helper_fill_var(info, &ifbdev->helper, sizes->fb_width, sizes->fb_height);
 
+	/* If the object is shmemfs backed, it will have given us zeroed pages.
+	 * If the object is stolen however, it will be full of whatever
+	 * garbage was left in there.
+	 */
+	if (ifbdev->ifb.obj->stolen)
+		memset_io(info->screen_base, 0, info->screen_size);
+
 	/* Use default scratch pixmap (info->pixmap.flags = FB_PIXMAP_SYSTEM) */
 
 	DRM_DEBUG_KMS("allocated %dx%d fb: 0x%08x, bo %p\n",