Add writeback support for sc7280. This was validated with kms_writeback test case in IGT. Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com> Patchwork: https://patchwork.freedesktop.org/patch/535244/ Link: https://lore.kernel.org/r/20230503223905.24754-1-quic_abhinavk@quicinc.com Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>