msm: kgsl: Avoid racing against context delete while releasing contexts

While releasing contexts take a reference to context inside read rcu
lock to avoid racing against context deletion. This will avoid using
dangling context pointer in device_release_contexts.

Change-Id: I76e787f6dde5a324fec23e81829174bd28134c6c
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>