drm/msm/adreno: warn if chip revn is verified before being set

The commit 010c8bba ("drm: msm: adreno: Disable preemption on Adreno
510") tried to check GPU's revn before revn being set. Add WARN_ON_ONCE
to prevent such bugs from happening again. A separate helper is
necessary so that the warning is displayed really just once instead of
being displayed for each of comparisons.

Suggested-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/531705/


Signed-off-by: Rob Clark <robdclark@chromium.org>