powerpc/powernv: Avoid re-registration of imc debugfs directory

export_imc_mode_and_cmd() function which creates the debugfs interface
for imc-mode and imc-command, is invoked when each nest pmu units is
registered.

When the first nest pmu unit is registered, export_imc_mode_and_cmd()
creates 'imc' directory under `/debug/powerpc/`. In the subsequent
invocations debugfs_create_dir() function returns, since the directory
already exists.

The recent commit <c33d4423> (debugfs: make error message a bit
more verbose), throws a warning if we try to invoke
`debugfs_create_dir()` with an already existing directory name.

Address this warning by making the debugfs directory registration in
the opal_imc_counters_probe() function, i.e invoke
export_imc_mode_and_cmd() function from the probe function.

Signed-off-by: Anju T Sudhakar <anju@linux.vnet.ibm.com>
Tested-by: Nageswara R Sastry <nasastry@in.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191127072035.4283-1-anju@linux.vnet.ibm.com