OMAP3: cpuidle: change the power domains modes determination logic

The achievable power modes of the power domains in cpuidle
depends on the system wide 'enable_off_mode' knob in debugfs.
Upon changing enable_off_mode, do not change the C-states
'valid' field but instead dynamically restrict the power modes
when entering idle.

The C-states 'valid' field is just used to enable/disable some
C-states at init and shall not be changed later on.

Signed-off-by: Jean Pihet <j-pihet@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>