drm/panthor: Use existing OPP table if present

drm/panthor: Use existing OPP table if present

On SoCs where the GPU's power-domain is in charge of setting performance
levels, the OPP table of the GPU node will have already been populated
during said power-domain's attach_dev operation.

To avoid initialising an OPP table twice, only set the OPP regulator and
the OPPs from DT if there's no OPP table present.

Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Link: https://patch.msgid.link/20251017-mt8196-gpufreq-v8-4-98fc1cc566a1@collabora.com
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>