msm: kgsl: Add a power constraints interface

Add an interface to hint the user specified power
constraint. User can specify a particular GPU power level
or a bandwidth vote. The power constraints are per context
or command batch. Power constraints are only for hinting
and should be used judiciously.

Change-Id: Ic10a5fff4abc88c51cd45a7d7ac58c0d4e5dcb25
Signed-off-by: Suman Tatiraju <sumant@codeaurora.org>
Signed-off-by: Lucille Sylvester <lsylvest@codeaurora.org>