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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>