drm/amd/display: FEC check in timing validation

[ Upstream commit 7d56a154 ]

[Why]
disable/enable leads FEC mismatch between hw/sw FEC state.

[How]
check FEC status to fastboot on/off.

Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Chiawen Huang <chiawen.huang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>