drm/nouveau: Fix potential memory leak in nouveau_drm_load()

We forget to free drm in all instances of failure, and additionally also
forget to destroy the master client if the other client fails
initialization.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>