msm: kgsl: Do not dump the istore on A3XX cores

The instruction store on A3XX GPU cores isn't really an instruction store
as on A2XX and it doesn't give us any debugging value. On top of all
that it can be unstable to read in some situations. Since it is of
questionable value and questionable stability pull it from the snapshot
for all A3XX cores and move the functionality to the A2XX specific code.

Change-Id: Ic0dedbada32d8b6b9b93768d148c2624faf3ef30
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>