drm/radeon/kms: ignore vga arbiter return.

Since we register all radeon devices, and the arbiter only cares about
VGA class ones, we will fail to startup on display controller class devices.
We don't gain anything by using the return value here.

this helps kms on sparc64 get started.

Reported-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>