drm/msm/dp: simplify dp_connector_get_modes()

Since dp_panel_get_modes() handling for dp_mode was removed,
dp_display_get_modes also doesn't change the passed dp_mode, drop the
unused dp_mode variable being allocated unused and then freed.

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/480137/
Link: https://lore.kernel.org/r/20220330223008.649274-3-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>