drm/amd/display: set dig fifo read start level to 7 before dig fifo reset

[Why]
DIG_FIFO_ERROR = 1 caused mst daisy chain 2nd monitor black.

[How]
We need to set dig fifo read start level = 7 before dig fifo reset during dig
fifo enable according to hardware designer's suggestion. If it is zero, it will
cause underflow or overflow and DIG_FIFO_ERROR = 1.

Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Wang Fudong <Fudong.Wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>