msm: kgsl: Delete the context event while the context is still valid

Make sure that the context is freed only after the event is deleted
from the list to avoid accessing invalid list pointers. This makes
sure that the context memory is not accessed after being freed.

Change-Id: I9196219f1472396c0063dbeaa5351f66e6e5648f
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>
Signed-off-by: Uma Maheshwari Bhiram <ubhira@codeaurora.org>