drm/msm/dpu: drop plane's default_scaling debugfs file

Proper support for the 'default_scaling' debugfs file was removed during
DPU driver pre-merge cleanup. Remove leftover file.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Link: https://lore.kernel.org/r/20211201222633.2476780-5-dmitry.baryshkov@linaro.org


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