drm/nv50/gr: add missing nv_error parameter priv

Commit ea7dce90

 ("drm/nv50/gr: print mpc trap name when it's not an mp
trap") added an nv_error call that was missing the priv parameter. This
causes GPFs if the error is ever hit.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>