drm/msm: Cleanup A6XX opp-level reading

The patch ("OPP: Add support for parsing the 'opp-level' property")
adds an API enabling a cleaner way to read the opp-level.  Let's use
the new API.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>