drm/omap: fix race condition with dev->obj_list

omap_gem_objects are added to dev->obj_list in omap_gem_new, and removed
in omap_gem_free_object. Unfortunately there's no locking for
dev->obj_list, which eventually leads to a crash:

WARNING: CPU: 1 PID: 1123 at lib/list_debug.c:59 __list_del_entry+0xa4/0xe0()
list_del corruption. prev->next should be e9281344, but was ea722b84

Add a spinlock to protect dev->obj_list.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
diff --git a/drivers/gpu/drm/omapdrm/omap_drv.h b/drivers/gpu/drm/omapdrm/omap_drv.h
index 57e11c1..b31c79f 100644
--- a/drivers/gpu/drm/omapdrm/omap_drv.h
+++ b/drivers/gpu/drm/omapdrm/omap_drv.h
@@ -105,6 +105,9 @@
 
 	struct workqueue_struct *wq;
 
+	/* lock for obj_list below */
+	spinlock_t list_lock;
+
 	/* list of GEM objects: */
 	struct list_head obj_list;