msm: msm_bus: Add API to change client context

Allow a client to change its bus bandwidth context without requiring
the client to unregister and re-register. The new API will be more
effecient at changing this context rather than doing the unregister
and re-register.

Change-Id: Ic075885127c49e183d81dd677faa32a12b3a21b4
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>