drm/mediatek: convert dpi driver to use drm_of_find_panel_or_bridge

drm/mediatek: convert dpi driver to use drm_of_find_panel_or_bridge

Convert dpi driver to use drm_of_find_panel_or_bridge.
This changes some error messages to debug messages (in the graph core).
Graph connections are often "no connects" depending on the particular
board, so we want to avoid spurious messages. Plus the kernel is not a
DT validator.
related links:
[1] https://lkml.org/lkml/2017/2/3/716
[2] https://lkml.org/lkml/2017/2/3/719

Signed-off-by: chunhui dai <chunhui.dai@mediatek.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>