drm/msm: register for OPP notifications

Device OPP's have a list of frequencies and voltages available.
DEVFREQ choose an operating frequency based on OPP's list and
DEVFREQ policy. OPP can be changed e.g. enabled or disabled in
case the thermal daemon wants to mitigate. In such scenarios we
want to correctly update the device frequency from the currently
active OPP levels. This patch registers for OPP notifications and
then calls update_devfreq() to let the DEVFREQ recommend the right
frequency.

Change-Id: I648e8641355a3728ffa9b12396e3287137d4c7ce
Signed-off-by: Sharat Masetty <smasetty@codeaurora.org>