drm/msm: Fix A6XX support for opp-level

The bindings for Qualcomm opp levels changed after being Acked but
before landing.  Thus the code in the GPU driver that was relying on
the old bindings is now broken.

Let's change the code to match the new bindings by adjusting the old
string 'qcom,level' to the new string 'opp-level'.  See the patch
("dt-bindings: opp: Introduce opp-level bindings").

NOTE: we will do additional cleanup to totally remove the string from
the code and use the new dev_pm_opp_get_level() but we'll do it in a
future patch.  This will facilitate getting the important code fix in
sooner without having to deal with cross-maintainer dependencies.

This patch needs to land before the patch ("arm64: dts: sdm845: Add
gpu and gmu device nodes") since if a tree contains the device tree
patch but not this one you'll get a crash at bootup.

Fixes: 4b565ca5 ("drm/msm: Add A6XX device support")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>