media: atomisp: use regulator_get_optional() for first attempt

Some BIOSes seem to use different names for some regulators.

Use regulator_get_optional() for the first attempt, in order
to avoid using the dummy regulator and produce a warning, in
the case that the first attempt fails.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>