drm/nouveau/kms/nv50: perform null check on msto[i] rathern than msto

The null check on the array msto is incorrect since msto is never
null. The null check should be instead on msto[i] since this is
being dereferenced in the call to drm_mode_connector_attach_encoder.

Thanks to Emil Velikov for pointing out the mistake in my original
fix and for suggesting the correct fix.

Detected by CoverityScan, CID#1375915 ("Array compared against 0")

Fixes: f479c0ba

 ("drm/nouveau/kms/nv50: initial support for DP 1.2 multi-stream")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>