drm/bridge: thc63lvd1024: Fix regulator_get_optional() misuse

The thc63lvd1024 driver requests a supply using regulator_get_optional()
but both the name of the supply and the usage pattern suggest that it is
being used for the main power for the device and is not at all optional
for the device for function, there is no handling at all for absent
supplies.  Such regulators should use the vanilla regulator_get()
interface, it will ensure that even if a supply is not described in the
system integration one will be provided in software.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>