drm/msm/adreno: Add helper for formating chip-id

This is used in a few places, including one that is parsed by userspace
tools.  So let's standardize it a bit better.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/549774/