drm/panfrost: Add missing include

Fix sparse warnings:
drivers/gpu/drm/panfrost/panfrost_devfreq.c:133:5:
 warning: symbol 'panfrost_devfreq_init' was not declared. Should it be static?
drivers/gpu/drm/panfrost/panfrost_devfreq.c:168:6:
 warning: symbol 'panfrost_devfreq_resume' was not declared. Should it be static?
drivers/gpu/drm/panfrost/panfrost_devfreq.c:182:6:
 warning: symbol 'panfrost_devfreq_suspend' was not declared. Should it be static?
drivers/gpu/drm/panfrost/panfrost_devfreq.c:212:6:
 warning: symbol 'panfrost_devfreq_record_transition' was not declared. Should it be static?

Signed-off-by: Steven Price <steven.price@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/7c9d75dd-ec67-4491-ca0c-79743211f308@arm.com