drm/amd/display: Add dcn401 DIG fifo enable/disable

drm/amd/display: Add dcn401 DIG fifo enable/disable

[Why]
Found while hotplugging MST daisy chain displays. Changing dispclk
during this sequence caused SMU hang due to DIG fifo not being disabled
correctly (caused by missing functions).

[How]
Adding disable/enable DIG fifo functions for dcn401

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Relja Vojvodic <relja.vojvodic@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>