drm/mediatek: dpi: Add dual edge sample mode support

To support RGB888_2X12_LE and RGB888_2X12_BE, dpi should sample
on dual edge which could recude half data io pins.
For RGB888_1X24, dpi just sample on single edge (rising or falling
edge).

Signed-off-by: Jitao Shi <jitao.shi@mediatek.com>
Signed-off-by: Rex-BC Chen <rex-bc.chen@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>