Move the CEC device parsing logic to a new function called
mtk_hdmi_get_cec_dev(), and move the parsing action to the end
of mtk_hdmi_dt_parse_pdata(), allowing to remove gotos in this
function, reducing code size and improving readability.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>