drm/msm/adreno: Don't capture register values if target doesn't define them

If the GPU target doesn't define a list of registers then gracefully skip
capturing and/or printing them. This is used by more complex targets like
6xx that have other means of capturing register values.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>