drm/amd/display: Preserve eDP mode on initial ASSR failure

Commit 56d8ce9d8c17 ("drm/amd/display: Apply correct panel mode when
reinitializing hardware") made ASSR failure fall back to the default panel
mode unless eDP mode had previously been applied.

However, dc_link is zero-initialized and DP_PANEL_MODE_DEFAULT is zero.
Before the first call to dp_set_panel_mode(), panel_mode therefore looks
like a previously applied default mode. If ASSR setup fails during the
first link training attempt, the driver incorrectly trains the eDP link
using the default scrambling mode.

On a Google Vilboz Chromebook running self-built coreboot firmware and
PSP verstage, this leaves the panel mostly black with corrupted output
along the top edge.

Track whether panel_mode has actually been initialized, and only consult
the saved mode after it has been applied. This retains the recovery
behavior while preserving eDP mode during initial link training.

Fixes: 56d8ce9d8c17 ("drm/amd/display: Apply correct panel mode when reinitializing hardware")
Assisted-by: Pi:gpt-5.6-sol
Reviewed-by: George Zhang <george.zhang@amd.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit ccb6c2702033f1002b45807fb3cc938760f26f78)
Cc: stable@vger.kernel.org # 6.4.x

0 files changed, 0 insertions, 0 deletions