msm: kgsl: Use a consistent API to change power state

Provide a simple API to change power state.  No need for
the driver to deal with power resources in detail outside of
the power specific files.  Just request the appropriate state
change and check to see if it took effect in cases where that
matters.

Change-Id: I80e92d48599339490e98337b319f784617245de4
Signed-off-by: Lucille Sylvester <lsylvest@codeaurora.org>
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>