drm/msm/a6xx: Remove both GBIF and RBBM GBIF halt on hw init

drm/msm/a6xx: Remove both GBIF and RBBM GBIF halt on hw init

Currently we're only deasserting REG_A6XX_RBBM_GBIF_HALT, but we also
need REG_A6XX_GBIF_HALT to be set to 0.

This is typically done automatically on successful GX collapse, but in
case that fails, we should take care of it.

Also, add a memory barrier to ensure it's gone through before jumping
to further initialization.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/542760/
Signed-off-by: Rob Clark <robdclark@chromium.org>