mlx4: remove leftover idr_pre_get() call

Commit 6a920060

 ("IB/mlx4: convert to idr_alloc()") forgot to remove
idr_pre_get() call in mlx4_ib_cm_paravirt_init().  It's unnecessary and
idr_pre_get() will soon be deprecated.  Remove it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jack Morgenstein <jackm@dev.mellanox.co.il>
Cc: Or Gerlitz <ogerlitz@mellanox.com>
Cc: Roland Dreier <roland@purestorage.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>