drm/radeon: fix race condition UAF in radeon_gem_set_domain_ioctl

Userspace can race to free the gobj(robj converted from), robj should not
be accessed again after drm_gem_object_put, otherwith it will result in
use-after-free.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Min Li <lm0963hack@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>