drm: simplify drm_*_set_unique()

Lets use kasprintf() to avoid pre-allocating the buffer. This is really
nothing to optimize for speed and the input is trusted, so kasprintf() is
just fine.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>