msm: kgsl: Add event to complete context detachment

Add an event with a callback to complete the detachment of context.
This allows the context destroy ioctl to return early without
waiting for the GPU to complete the last command submitted by
the context.

Change-Id: I2cce962f3260877906cbdf95a1b894bd2617d988
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>