ARM: OMAP2+: powerdomain: pwrdm_for_each_clkdm iterate only valid clkdms

No need to invoke callback when the clkdm pointer is NULL.

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Kevin Hilman <khilman@linaro.org>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>