drm/amd/display: Fixing DIG FIFO Error

[Why & How]
DIG_FIFO_READ_START_LEVEL should only be set to default value (7) by software.
Removed all instances of resetting the register to 0

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Pavle Kotarac <Pavle.Kotarac@amd.com>
Signed-off-by: Leo Chen <sancchen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>