video: fbdev: omap2: Remove deprecated regulator_can_change_voltage() usage

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.
As there is no practical use of it it can be removed.
At this point the regulator_set_voltage() calls can not be removed as the
DT data need to be fixed first.

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