drm/panfrost: Add support for multiple power domains

When there is a single power domain per device, the core will
ensure the power domain is switched on (so it is technically
equivalent to having not power domain specified at all).

However, when there are multiple domains, as in MT8183 Bifrost
GPU, we need to handle them in driver code.

Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200207052627.130118-6-drinkcat@chromium.org