drm/radeon/kms/combios: verify dac_adj values are valid

Some vbios dac_adj tables are all zeros.  Check for that
case and use the default table if so.

Should fix fdo bug 27478.

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