regulator: regulator_enable() permission checking

The regulator_enable() code wasn't actually checking that the
machine constraints had given permission to enable the regulator.
Add code to do that, but only if the regulator is not already on
due to something like always_on or being left on at startup since
in those cases there's no physical change being introduced and the
constraint wouldn't make any sense.

Also add matching code for disable(). We need to do less there since
either regulator_enable() should have succeeded first or the board
setup makes no sense.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>