drm/amd/display: Add null check to dml21_find_dc_pipes_for_plane

When a phantom stream is in the process of being deconstructed, there
could be pipes with no associated planes.  In that case, ignore the
phantom stream entirely when searching for associated pipes.

Cc: stable@vger.kernel.org
Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>