msm: kgsl: Add a check to free the kgsl_timeline object.

Add a check to free the kgsl_timeline object only after
destroy flag is set. This is to avoid further references to timeline
from kgsl if it is already freed and also to catch any unbalanced
kref count.

Change-Id: I8017b9a928a26b703eb92b17ab6a361e4b479e5a
Signed-off-by: Rajesh Kemisetti <rajeshk@codeaurora.org>