drm/vmwgfx: Convert screen targets to new helpers v3

Also implements the missing readback function and
fixes page flip in case of no event.

v2:
- Adapt to the work done for screen targets for 2d, in particular
Handle proxy surface updates.
- Remove execbuf quirks since we now use fifo reserve / commit.
- Revert the initial placement of vmw dma buffers.

v3: Address review comments.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>

drivers/gpu/drm/vmwgfx/vmwgfx_resource.c

@@ -497,7 +497,7 @@ int vmw_user_dmabuf_alloc(struct vmw_private *dev_priv,
 
 	ret = vmw_dmabuf_init(dev_priv, &user_bo->dma, size,
 			      (dev_priv->has_mob) ?
-			      &vmw_mob_placement :
+			      &vmw_sys_placement :
 			      &vmw_vram_sys_placement, true,
 			      &vmw_user_dmabuf_destroy);
 	if (unlikely(ret != 0))