drm/radeon/kms: Fix r600 blit cleanup path

r600 blit cleanup path need to check if a bo was allocated before
trying to free or unpin it. This patch add this check and avoid
oops when the initialization on r6xx or r7xx hw fails.

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