drm/vmwgfx: Send the correct nonblock option for atomic_commit

Page flip can be slow for vmwgfx in some cases, like need to do surface
copy to different surface or waiting for IN_FENCE_FD. Enabling
nonblocking commits for vmwgfx in case userspace request it.

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