drm/amd/display: contional remove disable dig_fifo when blank

keep dig_fifo enable for test pattern generation.

Tested-by: Daniel Wheeler <Daniel.Wheeler@amd.com>
Reviewed-by: Chris Park <Chris.Park@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>