drm/radeon/kms: don't fail if we fail to init GPU acceleration

Userspace can query if acceleration is working or not true get
info ioctl and could fallback to software if for some reason
kernel failed to initialize KMS. This should allow to give a
working KMS setup in all case (even with non functionning accel).

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>