drm/panfrost: devfreq: Disable devfreq when num_supplies > 1

GPUs with more than a single regulator (e.g. G72 on MT8183) will
require platform-specific handling for devfreq, for 2 reasons:
 1. The opp core (drivers/opp/core.c:_generic_set_opp_regulator)
    does not support multiple regulators, so we'll need custom
    handlers.
 2. Generally, platforms with 2 regulators have platform-specific
    constraints on how the voltages should be set (e.g.
    minimum/maximum voltage difference between them), so we
    should not just create generic handlers that simply
    change the voltages without taking care of those constraints.

Disable devfreq for now on those GPUs.

Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210421132841.v13.3.I3af068abe30c9c85cabc4486385c52e56527a509@changeid