msm: kgsl: Only allow two counters for VBIF performance counters

There are only two VBIF counter groups so validate that the user
doesn't pass in > 1 and clean up the if/else clause.

Change-Id: Ic0dedbad3d5a54e4ceb1a7302762d6bf13b25da1
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>