drm/msm/dp: remove unused stubs

Refactoring DP code transformed several functions into empty stubs.
Remove them.

Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/490100/
Link: https://lore.kernel.org/r/20220617232434.1139950-1-dmitry.baryshkov@linaro.org


Signed-off-by: Rob Clark <robdclark@chromium.org>