drm/amd/display: Don't return false if no stream

pipe_ctx[i] exists even if the pipe is not
in use. If the pipe is not in use it will
always have a null stream, so don't return
false in this case.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>