drm/nv50: fix ramin heap size for kernel channel too

Port change from "drm/nouveau: Keep RAMIN heap within the channel"
to kernel channel, which has its own ramin heap initialisation.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: Younes Manton <younes.m@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>