drm/i915: Hook connector to encoder during load detection (fixes tv/vga detect)

With the DRM-driven DPMS code, encoders are considered idle unless a
connector is hooked to them, so mode setting is skipped. This makes load
detection fail as none of the hardware is enabled.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Eric Anholt <eric@anholt.net>