OMAP3: cpuidle: Update statistics for correct state

When 'enable_off_mode' is 0, the target power state for MPU
and CORE was locally changed to PWRDM_POWER_RET but, the
statistics are updated for idle state originally selected
by the governor.

This patch 'invalidates' the idle states that lead either of
MPU or Core to PWRDM_POWER_OFF state when 'enable_off_mode'
is '0'. The states are valid once 'enable_off_mode' is set
to '1'.

Added function next_valid_state() to check if current state
is valid; else get the next valid state. It is called from
omap3_enter_idle_bm().

Signed-off-by: Sanjeev Premi <premi@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>