drm/msm/a6xx: Enable preemption for tested a7xx targets

Initialize with 4 rings to enable preemption.

Add the "preemption_enabled" module parameter to override this.

Tested-by: Rob Clark <robdclark@gmail.com>
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8650-QRD
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8550-QRD
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8450-HDK
Signed-off-by: Antonino Maniscalco <antomani103@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/618029/


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