msm: kgsl: Do not save context if it is being destroyed

No need to save a context if it is being destroyed since
it will no longer be used at all. This is better for
performance and also avoids the use of legacy kgsl
context save code for contexts that use preambles

Change-Id: I19a64e82188b4132f353bb61c21e4ed2281092fc
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>