drm/msm/dpu: pull the is_cmd_mode out of _dpu_encoder_update_vsync_source()

Setting vsync source makes sense only for DSI CMD panels. Pull the
is_cmd_mode condition out of the function into the calling code, so that
it becomes more explicit.

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Patchwork: https://patchwork.freedesktop.org/patch/598736/
Link: https://lore.kernel.org/r/20240613-dpu-handle-te-signal-v2-4-67a0116b5366@linaro.org