mlx4_core: Move table_find from fmr_alloc to fmr_enable

mlx4_table_find (for FMR MPTs) requires that ICM memory already be
mapped.  Before this fix, FMR allocation depended on ICM memory
already being mapped for the MPT entry.  If all currently mapped
entries are taken, the find operation fails (even if the MPT ICM table
still had more entries, which were just not mapped yet).

This fix moves the mpt find operation to fmr_enable, to guarantee that
any required ICM memory mapping has already occurred.

Found by Oren Duer of Mellanox.

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>