drm/radeon/kms: Fix retrying ttm_bo_init() after it failed once.

If ttm_bo_init() returns failure, it already destroyed the BO, so we need to
retry from scratch.

Signed-off-by: Michel Dänzer <daenzer@vmware.com>
Tested-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>