msm: kgsl: Cancel events of a context when destroying it

When a kgsl context is destroyed, make sure that its events are also
canceled. Not doing this may result in a kernel panic if the callback
is run on an invalid context.

Change-Id: Ic11119bffc88834c66d9985e19f15c32a3236256
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>