drm/radeon/kms: add disabled vbios accessor for NI asics

Some systems disable the vbios on secondary cards or cards that
have been posted.  This code re-enabled the vbios so the driver
can load it.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>