drm/omap: panel-dsi-cm: use bulk regulator API

Use bulk regulator API to simplify the code. This also switches
from _optional variant to normal variant, which will provide a
dummy regulator (i.e. if some always-enabled regulator is not
described in DT).

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-23-tomi.valkeinen@ti.com