drm/panfrost: Remove NULL checks for regulator

devm_regulator_get() is now used to populate pfdev->regulator which
ensures that this cannot be NULL (a dummy regulator will be returned if
necessary). So remove the checks 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/3e3a2c8a-b4fc-8af6-39e1-b26160db2c7c@arm.com