drm/msm/a6xx: Skip empty protection ranges entries

Some specific SKUs leave certain protection range registers empty.
Allow for that behavior.

Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/543334/


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