drm/amd/display: Don't consider cursor for no plane case in DML1

[Description]
For no plane scenarios we should not consider cursor as there cannot
be any cursor if  there's no planes. This fixes an issue where
dc_commit_streams fails due to prefetch bandwidth requirements
(the display config + dummy planes + cursor causes the prefetch
bandwidth to exceed what is possible).

Reviewed-by: Chaitanya Dhere <chaitanya.dhere@amd.com>
Signed-off-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>