msm: kgsl: Pass the rb pointer when changing current context

Make the framework to pass the ringbuffer pointer when we change
the current context. This serves as the basis to support switching
contexts at different priority levels. The ringbuffer passed is the
rb which the context uses to submit commands.

Change-Id: Ifdf0a5dfb967690cacd11e758a94571da25667be
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>