msm: kgsl: Fix some variable issues in the perfcounter code

Fix two potential issues in the A3XX performance counter code:

* Some compilers might get worried that the 'in' variable might be
  used uninitialized in the perfcounter read functions. It isn't;
  but not really worth arguing over.

* It is possible for the result of the macro ADRENO_PERFCOUNTERS to
  be NULL if the GPU device isn't define.  Correctly check for a
  NULL pointer to avoid a dereference.

Change-Id: Ic0dedbad5a74101233ced2402e98e36d1acb2d71
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>