drm/msm/dsi: parse vsync source from device tree

Allow board's device tree to specify the vsync source (aka TE source).
If the property is omitted, the display controller driver will use the
default setting.

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
[DB: fixed clearing of return value if there is no TE property]
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Patchwork: https://patchwork.freedesktop.org/patch/598740/
Link: https://lore.kernel.org/r/20240613-dpu-handle-te-signal-v2-6-67a0116b5366@linaro.org