drm/omap: Remove regulator API abuse

regulator_can_change_voltage() is deprecated and it's use is not necessary
as commit:
6a0028b3 regulator: Deprecate regulator_can_change_voltage()
describers it clearly.

Also, regulator_set_voltage() is misused in the driver, as it is
supposed to be used only in cases where the regulator voltage needs to
be changed dynamically at runtime. In DSS's case, we always want a fixed
voltage, set in the .dts files.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>