drm/msm/dpu: simplify interface flush handling

Instead of calling 4 callbacks to set pending masks, call just one to
update both pending_flush_mask and pending_intf_flush mask. Note, that
CMD mode support incorrectly did not update pending_intf_flush mask,
breaking CMD support on SC7180/SM8x50.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>