drm/panfrost: dynamically alloc regulators

We will later introduce regulators managed by OPP.

Only alloc regulators when it's needed. This also help use
to release the regulators only when they are allocated.

Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Signed-off-by: Clément Péron <peron.clem@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200710095409.407087-10-peron.clem@gmail.com