drm/msm: reduce usage of round_pixclk callback

drm/msm: reduce usage of round_pixclk callback

The round_pixclk() callback returns different rate only on MDP4 in HDMI
(DTV) case. Stop using this callback in other cases to simplify
mode_valid callbacks.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Link: https://lore.kernel.org/r/20220106070656.482882-1-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>