drm/msm/dpu: add writeback support for sc7180

Add writeback support for sc7180 devices. This has been
validated on sc7180 chromebook using IGT writeback test
suite.

localhost /usr/local/libexec/igt-gpu-tools # ./kms_writeback
Starting subtest: writeback-pixel-formats
Subtest writeback-pixel-formats: SUCCESS (0.001s)
Starting subtest: writeback-invalid-parameters
Subtest writeback-invalid-parameters: SUCCESS (0.004s)
Starting subtest: writeback-fb-id
Subtest writeback-fb-id: SUCCESS (0.009s)
Starting subtest: writeback-check-output
Subtest writeback-check-output: SUCCESS (0.203s)

Adding this will increase IGT coverage on chromebooks and also
allow using writeback functionality for other use-cases as
necessary.

Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/485787/
Link: https://lore.kernel.org/r/20220510235439.31916-1-quic_abhinavk@quicinc.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>