drm/vmwgfx: Use the standard atomic helpers for page-flip

Our wrappers don't do anything useful anymore except calling the
atomic helpers.

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