msm: kgsl: Update thermal level via OPP notifier

Subscribes to OPP framework OPP_EVENT_ENABLE and
OPP_EVENT_DISABLE events. The events will be generated
when thermal engine throttling GPU frequency via
disabling/enabling GPU OPP. KGSL uses the event to
update its internal thermal power level data. The data
will be used to ensure no GPU clients, especially CL
applications request higher GPU Frequency than thermal
engine allows.

Change-Id: Iee86ef8060878942d180a2f313da3f4226123d54
Signed-off-by: George Shen <sqiao@codeaurora.org>