msm: kgsl: Read snapshot registers in the target specific functions

Read the common GPU status registers in the target specific snapshot
functions rather than using adreno_readreg() to read them from the
generic code.

Change-Id: Ic0dedbad17f8744827fda3ce5a035f3bc913ea3f
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>