PM / devfreq: tegra: Drop primary interrupt handler

There is no real need in the primary interrupt handler, hence move
everything to the secondary (threaded) handler. In a result locking
is consistent now and there are no potential races with the interrupt
handler because it is protected with the devfreq's mutex.

Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>