drm/amd/display: Apply correct panel mode when reinitializing hardware

[Why]
When link training during engine recovery, ASSR might fail causing panel
mode to be reset to default. This should not happen for eDP as it
will prevent the panel from turning back on.

[How]
Added dp_panel_mode to struct dc_link to remember previously applied
panel mode. Do not reset panel mode to default while performing link
training if previously used panel mode = eDP.

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Michael Mityushkin <michael.mityushkin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>