msm: kgsl: Add context unregister HFI

This is a special sequence because we don't want to wait for
the ack back while holding the device mutex. This means, we
have to define a new sequence of unlocking the device mutex
before we begin our wait for ack.

Change-Id: I3ea27bdccd66cdaafd8cff6cc1366848c1a4bea6
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>