omap: pm: Move set_pwrdm_state routine to common pm.c

The set_pwrdm_state() is needed on omap4 as well so move
this routine to common pm.c file so that it's available for omap3/4

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>