media: qcom: camss: Convert to per-VFE pointer for power-domain linkages

Right now we use the top-level camss structure to provide pointers via
VFE id index back to genpd linkages.

In effect this hard-codes VFE indexes to power-domain indexes in the
dtsi and mandates a very particular ordering of power domains in the
dtsi, which bears no relationship to a real hardware dependency.

As a first step to rationalising the VFE power-domain code and breaking
the magic indexing in dtsi use per-VFE pointers to genpd linkages.

The top-level index in msm_vfe_subdev_init is still used to attain the
initial so no functional or logical change arises from this change.

Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Tested-by: Matti Lehtimäki <matti.lehtimaki@gmail.com>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>