OMAP: DSS2: Remove suspicous and unused TAAL regulator API usage

OMAP: DSS2: Remove suspicous and unused TAAL regulator API usage

The TAAL driver contains some regulator support which is currently unused
(the code is there but the one panel supported by the driver doesn't have
any regulators provided). This code mostly looks like an open coded
version of the regulator core bulk API.

The only additional feature is that a voltage range can be set once when
the device is opened, though this is never varied at runtime. The general
expectation is that if the device is not actively managing the voltage of
the device (eg, doing DVFS) then any configuration will be done using the
constraints rather than by drivers, saving them code and ensuring that
they work well with systems where the voltage is not configurable.

If systems are added needing regulator support this can be added back in,
though it should be based on core features rather than open coding things.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>