drm/msm/adreno: Check for zap node availability

This should allow disabling the zap node via an overlay, for slbounce.

Suggested-by: Nikita Travkin <nikita@trvn.ru>
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Bjorn Andersson <quic_bjorande@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/594572/