drm/msm/dpu: support setting the TE source

Make the DPU driver use the TE source specified in the DT. If none is
specified, the driver defaults to the first GPIO (mdp_vsync_p).

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/598733/
Link: https://lore.kernel.org/r/20240613-dpu-handle-te-signal-v2-7-67a0116b5366@linaro.org