drm/panfrost: Add missing check for pfdev->regulator

When modifying panfrost_devfreq_target() to support a device without a
regulator defined I missed the check on the error path. Let's add it.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: e21dd290

 ("drm/panfrost: Enable devfreq to work without regulator")
Signed-off-by: Steven Price <steven.price@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190822093218.26014-1-steven.price@arm.com