drm/mediatek: dp: Add support for embedded DisplayPort aux-bus

drm/mediatek: dp: Add support for embedded DisplayPort aux-bus

For the eDP case we can support using aux-bus on MediaTek DP: this
gives us the possibility to declare our panel as generic "panel-edp"
which will automatically configure the timings and available modes
via the EDID that we read from it.

To do this, move the panel parsing at the end of the probe function
so that the hardware is initialized beforehand and also initialize
the DPTX AUX block and power both on as, when we populate the
aux-bus, the panel driver will trigger an EDID read to perform
panel detection.

Last but not least, since now the AUX transfers can happen in the
separated aux-bus, it was necessary to add an exclusion for the
cable_plugged_in check in `mtk_dp_aux_transfer()` and the easiest
way to do this is to simply ignore checking that when the bridge
type is eDP.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: Chen-Yu Tsai <wenst@chromium.org>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20230725073234.55892-10-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>