msm: kgsl: Dump preemption record only if it is enabled

During GPU snapshot dump path, a5xx_snapshot() tries to
capture preemption record without really checking on the
availability of preemption feature.

Add a check for preemption feature and then proceed.

Change-Id: I6ee23a1d8006ba18a25fb341bb88d8944ff054b7
Signed-off-by: Rajesh Kemisetti <rajeshk@codeaurora.org>