drm/bridge: analogix_dp: Set PD_INC_BG first when powering up edp phy

Following the correct power up sequence:
dp_pd=ff => dp_pd=7f => wait 10us => dp_pd=00

Cc: Stéphane Marchesin <marcheu@chromium.org>
Signed-off-by: zain wang <wzz@rock-chips.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180423105003.9004-7-enric.balletbo@collabora.com