drm/nve0/fifo: allocate usermem as needed

Memory was always allocated for 4096 channels. Change this to allocate
what we actually need according to the number of channels we use.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>