msm: kgsl: Use the standard functions to read the queued timestamp

Use the standard KGSL functions to access the queued timestamp in the
generic event code rather than directly dereference the adreno_device.

Change-Id: Ic0dedbadfc3004ba9bb26dc6bda82dfe61663106
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>