drm/panfrost: Remove NULL check for regulator

devm_regulator_get() is used to populate pfdev->regulator which ensures
that this cannot be NULL (a dummy regulator will be returned if
necessary). So remove the check in panfrost_devfreq_target().

Signed-off-by: Steven Price <steven.price@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191004144413.42586-1-steven.price@arm.com