drm/radeon/kms: fix oops when set_base is call with no FB

Just do nothing if crct_set_base() is called with no FB.

The oops happens when the user switches between X & vt or in some case
when changing mode.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>