drm/msm/dpu: limit wb modes based on max_mixer_width

As explained in [1], using max_linewidth to limit the modes
does not seem to remove 4K modes on chipsets such as
sm8250 where the max_linewidth actually supports 4k.

This would have been alright if dual SSPP support was
present but otherwise fails the per SSPP bandwidth check.

The ideal way to implement this would be to filter out
the modes which will exceed the bandwidth check by computing
it.

But this would be an exhaustive solution till we have
dual SSPP support.

Let's instead use max_mixer_width to limit the modes.

max_mixer_width still remains 2560 on sm8250 so even if
the max_linewidth is 4096, the only way 4k modes could have
been supported is to have source split enabled on the SSPP.

Since source split support is not enabled yet in DPU driver,
enforce max_mixer_width as the upper limit on the modes.

[1] https://patchwork.freedesktop.org/patch/489662/

Fixes: e67dcecd

 ("drm/msm/dpu: limit writeback modes according to max_linewidth")
Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/489893/
Link: https://lore.kernel.org/r/1655407606-21760-1-git-send-email-quic_abhinavk@quicinc.com
Signed-off-by: Rob Clark <robdclark@chromium.org>