drm/panthor: call into devfreq for current frequency

drm/panthor: call into devfreq for current frequency

As it stands, panthor keeps a cached current frequency value for when it
wants to retrieve it. This doesn't work well for when things might
switch frequency without panthor's knowledge.

Instead, implement the get_cur_freq operation, and expose it through a
helper function to the rest of panthor.

Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Karunika Choo <karunika.choo@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-v8-3-98fc1cc566a1@collabora.com
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>