venus: pm_helpers: Control core power domain manually

Presently we use device_link to control core power domain. But this
leads to issues because the genpd doesn't guarantee synchronous on/off
for supplier devices. Switch to manually control by pmruntime calls.

Tested-by: Fritz Koenig <frkoenig@chromium.org>
Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>