msm: kgsl: Assign a valid context only after one has been restored

Assign a context active context only after that context has been
restored. This is required because Code section to change
pagetables or flush tlb in stream require a valid context to be
active.

Change-Id: I513cf235adf2bde04af47944950d58ba1af1f61f
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>