drm/amd/display: Fix check for stream and plane

[WHY]
Function wasn't returning false when it had a no stream

[HOW]
Made it return false when it had no stream.

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