drm/nouveau: completely fail init if we fail to map the PRAMIN BAR

On cards where there's a specific BAR for PRAMIN, we used to try and fall
back to the "legacy" aperture within the mmio BAR.

This is doomed to cause problems, so lets just fail completely as there's
obviously something else very wrong anyway.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>