drm/msm/dp: fix panel bridge attachment

In commit 8a3b4c17 ("drm/msm/dp: employ bridge mechanism for display
enable and disable") the DP driver received a drm_bridge instance, which
is always attached to the encoder as a root bridge. However it conflicts
with the panel_bridge support for eDP panels. The panel bridge attaches
to the encoder before the "dp" bridge (DP driver's drm_bridge instance
created in msm_dp_bridge_init()) has a chance to do so. Change
panel bridge attachment to come after the "dp" bridge attachment (and to
use it as a previous bridge).

Fixes: 8a3b4c17

 ("drm/msm/dp: employ bridge mechanism for display enable and disable")
Cc: Kuogee Hsieh <quic_khsieh@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Tested-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20220211224006.1797846-2-dmitry.baryshkov@linaro.org


[db: fixed commit message according to Stephen's suggestions]
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>