drm/msm/dsi: drop modeset sanity checks

Drop the overly defensive modeset sanity checks of function parameters
which have already been checked or used by the callers.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Tested-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
Reviewed-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/502678/
Link: https://lore.kernel.org/r/20220913085320.8577-11-johan+linaro@kernel.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>