drm/amd/display: Defer DIG FIFO disable after VID stream enable

[Why]
On some monitors we see a brief flash of corruption during the
monitor disable sequence caused by FIFO being disabled in the middle
of an active DP stream.

[How]
Wait until DP vid stream is disabled before turning off the FIFO.

The FIFO reset on DP unblank should take care of clearing any FIFO
error, if any.

Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Syed Hassan <Syed.Hassan@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>