drm/vmwgfx: Fix prime with external buffers

In the Linux kernel, the following vulnerability has been resolved: drm/vmwgfx: Fix prime with external buffers Make sure that for external buffers mapping goes through the dma_buf interface instead of trying to access pages directly. External buffers might not provide direct access to readable/writable pages so to make sure the bo's created from external dma_bufs can be read dma_buf interface has to be used. Fixes crashes in IGT's kms_prime with vgem. Regular desktop usage won't trigger this due to the fact that virtual machines will not have multiple GPUs but it enables better test coverage in IGT.