PM / devfreq: Use lockdep asserts instead of manual checks for locked mutex

Instead of warning when mutex_is_locked(), just use the lockdep
framework.  The code is smaller and checks could be disabled for
production environments (it is useful only during development).

Put asserts at beginning of function, even before validating arguments.

The behavior of update_devfreq() is now changed because lockdep assert
will only print a warning, not return with EINVAL.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>