drm/vmwgfx: Validate the box size for the snooped cursor

In the Linux kernel, the following vulnerability has been resolved: drm/vmwgfx: Validate the box size for the snooped cursor Invalid userspace dma surface copies could potentially overflow the memcpy from the surface to the snooped image leading to crashes. To fix it the dimensions of the copybox have to be validated against the expected size of the snooped cursor.