OMAP3: PM: Use pwrdm_set_next_pwrst instead of set_pwrdm_state in idle loop

It is more efficient to use pwrdm_set_next_pwrst for mpu, core and neon
instead of set_pwrdm_state in idle loop. It is anyway known that those are
active in idle loop. So no need to use set_pwrdm_state.

Signed-off-by: Jouni Hogander <jouni.hogander@nokia.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>