drm/nouveau/bar/nv50: ensure BAR is mapped

commit f10b83de upstream.

If the BAR is zero size, it indicates it was never successfully mapped.
Ensure that the BAR is valid during initialization before attempting to
use it.

Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>