drm/mediatek: dpi/dsi: Change the getting possible_crtc way

For current mediatek dsi encoder, its possible crtc is fixed in crtc
0, and mediatek dpi encoder's possible crtc is fixed in crtc 1. In
some SoC the possible crtc is not fixed in this case, so search
pipeline information to find out the correct possible crtc.

Signed-off-by: Stu Hsieh <stu.hsieh@mediatek.com>
Signed-off-by: Jitao Shi <jitao.shi@mediatek.com>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>