drm/msm/dp: Simplify the dp_debug debugfs show function

drm/msm/dp: Simplify the dp_debug debugfs show function

The "dp_debug" show function allocates a buffer and piecemeal appends
line by line, checking for buffer overflows etc.

Migrate the function to seq_file, to remove all the extra book keeping
and simplify the function.

Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20211007190211.3158868-1-bjorn.andersson@linaro.org
Signed-off-by: Rob Clark <robdclark@chromium.org>