[media] mt9p031: Use bulk regulator API

The sensor is powered by three supplies. Use the bulk regulator API to
enable and disable them instead of performing the operations manually.
This fixes a warning caused by ignoring the return value of
regulator_enable().

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>