drm/amd/display: Clear FEC_READY shadow register if DPCD write fails

[why]
As a fail-safe, in case 'set FEC_READY' DPCD write fails, a HW shadow
register should be cleared and the internal FEC stat should be set to
'not ready'. This is to make sure HW settings will be consistent with
FEC_READY state on the RX.

Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Joshua Aberback <Joshua.Aberback@amd.com>
Acked-by: Chris Park <Chris.Park@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>