blk-mq: enable polling support by default

Now that applications need to explicitly ask for polling we can enable it
by default in blk-mq drivers.  Note that this will only have an affect
on driver that supply a poll function, which currently only includes nvme.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Stephen Bates <stephen.bates@pmcs.com>
Tested-by: Stephen Bates <stephen.bates@pmcs.com>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>