drm/i915: fix oops on single crtc devices.

(regression fix since fbdev/kms rework).

My fb rework didn't remember about the 84/65s.

Reported-by: Ondrej Zary <linux@rainbow-software.org>
Tested-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>