Everytime we run bridge detection and/or EDID read we run a poweron
and poweroff sequence for both the AUX and the panel; moreover, this
is also done when enabling the bridge in the .atomic_enable() callback.

Move this power on/off sequence to a new mtk_dp_aux_panel_poweron()
function as to commonize it.
Note that, before this commit, in mtk_dp_bridge_atomic_enable() only
the AUX was getting powered on but the panel was left powered off if
the DP cable wasn't plugged in while now we unconditionally send a D0
request and this is done for two reasons:
 - First, whether this request fails or not, it takes the same time
   and anyway the DP hardware won't produce any error (or, if it
   does, it's ignorable because it won't block further commands)
 - Second, training the link between a sleeping/standby/unpowered
   display makes little sense.
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: Chen-Yu Tsai <wenst@chromium.org>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20230725073234.55892-3-angelogioacchino.delregno@collabora.com/Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>