msm: kgsl: Get an active count before snapshotting GPU

Acquire an active count before attempting to capture a snapshot.
Doing so prevents instances of not being able to snapshot because of
switched off clocks.

CRs-fixed: 511878
Change-Id: Ib80b57c127566e274d4018001561f4da7ea38265
Signed-off-by: Harsh Vardhan Dwivedi <hdwivedi@codeaurora.org>