drm/vmgfx: kill off unused init_mutex

According to the docs - prevents firstopen/lastclose races. Yet never
used in practise.

Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190522164119.24139-2-emil.l.velikov@gmail.com

drivers/gpu/drm/vmwgfx/vmwgfx_drv.c

@@ -641,7 +641,6 @@ static int vmw_driver_load(struct drm_device *dev, unsigned long chipset)
 		INIT_LIST_HEAD(&dev_priv->res_lru[i]);
 	}
 
-	mutex_init(&dev_priv->init_mutex);
 	init_waitqueue_head(&dev_priv->fence_queue);
 	init_waitqueue_head(&dev_priv->fifo_queue);
 	dev_priv->fence_queue_waiters = 0;

drivers/gpu/drm/vmwgfx/vmwgfx_drv.h

@@ -484,11 +484,6 @@ struct vmw_private {
 
 	spinlock_t resource_lock;
 	struct idr res_idr[vmw_res_max];
-	/*
-	 * Block lastclose from racing with firstopen.
-	 */
-
-	struct mutex init_mutex;
 
 	/*
 	 * A resource manager for kernel-only surfaces and