drm/msm/dp: drop dp_mode argument from dp_panel_get_modes()

Since the commit ab205927

 ("drm/msm/dp: remove mode hard-coding in
case of DP CTS") the function dp_panel_get_modes() doesn't use (or fill)
the dp_mode argument. Drop it completely.

Reviewed-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/480138/
Link: https://lore.kernel.org/r/20220330223008.649274-2-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>