msm: kgsl: fix MIN power constraints in non-trivial cases

Switching from a MAX to a MIN power constraint in the same
context is broken.  So is switching from a MAX context constraint
to a MIN constraint on a different context.  Clarify constraint
ownership, update the timestamps accordingly, and reset constraints
when changed by the context.

Change-Id: Id27dbc20e2aac994101817af3857451c1703219e
Signed-off-by: Lucille Sylvester <lsylvest@codeaurora.org>