drm/msm/a6xx: Add some missing header definitions

Add a definition of the GMU_AHB_FENCE_STATUS_CLR reg and CP_PROTECT_CNTL
bitfields.

This may be substituted with a mesa header sync.

Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/543330/


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