drm/radeon: Don't unreserve twice on failure to validate.

This is done later in radeon_object_list_unvalidate(). Doing it twice triggers
a BUG in TTM, rendering X on KMS unusable until reboot.

Signed-off-by: Michel Dänzer <daenzer@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>